Mizuno’s Pro S-4 irons are the Signature Series’ missing piece

For the first time in more than half a decade, Mizuno has a true better-player cavity-back iron.

The Japanese sports giant, which has always been known for producing soft-feeling forged irons, has recently shed the idea that their irons are only for better players and become one of the market leaders in players distance and game improvement irons. In their current lineup, Mizuno has at least four different irons that could be categorized as players distance.

Meanwhile, the first two offerings in the new Mizuno Pro Signature Series, the S-1 and S-3, were a compact blade and a compact shallow cavity. Irons designed for elite ball strikers, but not as much for a player who wanted just a touch of forgiveness.

But now the company is adding to the Signature Series with the new Pro S-4, the largest and most forgiving cavity-back option in the lineup made up of all single-piece Grain Flow forged irons.

“The S-4 is a more forgiving, slightly larger, slightly more offset version of Mizuno Pro,” Mizuno Director of Golf Chris Voshall told GOLF. “Which all sound like things that are not Mizuno Pro. But it’s just very slight tweaks in all of those things.”

Keep reading below for more on the Mizuno Pro S-4 irons, including my take on the release.

What’s actually new with Mizuno Pro S-4 irons — and why you should care

Filling the cavity-back-sized hole

Mizuno branched off its Mizuno Pro line into the Signature Series — single-piece forgings for players who demand feel above all — and the Modern Series — for players who want Mizuno Pro feel, but want the speed boost from modern tech too.

While the S-1 and S-3 catered to the elite ball strikers, and M-13 and M-15 were for players seeking more distance, the S-4 is for the player in the middle: Players who want feel and forgiveness but don’t need extra speed. It’s a player that Voshall thinks has been overlooked in the industry, with all the range going toward players distance or players performance irons.

“It’s one that I think a lot of players who are the low single-digit handicaps are gonna really like,” he said. “The feel is there; the performance— it’s like sneakily forgiving.”

This was always going to be the idea with the Signature Series. Give good ball strikers several options that preserve feel, while allowing them to pick and choose certain features they like, like sole shape, workability and more.

Voshall admitted that this isn’t going to be an iron that flies off the shelf, but it’s also an iron that fits right into the existing Mizuno Pro lineup seamlessly as a combo set. He suggested an S-3 player moving to S-4 in the long irons, or an S-4 player moving to one of the M-Series irons in the long and mid-irons.

New shape, same tech

While the S-4 is a more forgiving Mizuno Pro Signature Series iron, it’s hardly just a larger version of the S-3.

Instead, the S-4 is a new shape that features a progressive offset flow and topline throughout the set. This helps the long irons inspire more confidence and keeps the short irons in a more compact profile for precise scoring.

While the S-3 has a straight muscle line along the back of the iron, the S-4 has a new curved muscle that strategically moves mass to the toe and heel, increasing stability and lowering the CG in addition to a wider sole. It also gives the iron a neutral bias.

“There’s no toe-heel bias to anything; the weighting of that cavity actually has the mass extending up in the toe and the heel,” Voshall said. “So it’s bringing the CG lower, it’s making the MOI a little bit larger, but all in a small kind of player’s package.”

But, just like the rest of the Mizuno Pro line, the S-4 features the same construction that fans of the brand have come to know and love. The iron is constructed from a single piece of 1025E Pure Select Mild Carbon Steel using Mizuno’s proprietary Grain Flow Forged HD process. The feel is further enhanced by a soft copper underlay and Harmonic Impact Technology.

The S-4 also borrows the Triple Cut sole that’s been a part of more recent Mizuno Pro irons such as the M-13 and S-3. The beveled leading edge and increased trailing edge relief help offset the wider sole.

The Mizuno Pro S-4

What is it: While the S-4 is the largest iron in the Mizuno Pro Signature family, it’s still a compact players iron designed for elite ball strikers.

Who it’s for: Players who want a single-piece forged feel, but more forgiveness without the additional speed of a larger iron.

My take: This is the Mizuno iron I’ve been waiting for

For a few years there, Mizuno offered its 223 and 243 cavity-back irons for better players who didn’t want to play blades. But those irons were a bit too jumpy for golfers (like me) who just wanted a little more forgiveness. Mizuno solved this by reprioritizing the new M-13 as a “speed cavity” and more in the players distance space.

When the S-3 came out, I thought this might have been the option I was looking for, but after playing them for a few months, I realized that the S-3 just didn’t offer the stability I needed. No knock on either of those irons, but they just weren’t for me.

With the Pro S-4, Mizuno finally offers the single-piece forged cavity-back that is easy for me to launch and gives me stability for the occasional mishit.

The Pro S-4 is the first iron that truly allows me to experience the Triple Cut sole properly. While the S-4’s sole is wider than the MP-18 MMCs I played a few years ago, it doesn’t play as wide because of the bevels.

At impact, the sole cuts through the turf quickly and makes me feel like I’m picking off a piece of turf the size of a slice of butter.

In the short testing I’ve done, I’ve found it very easy to work the ball up, down, draw, fade, or whatever I want, and as someone who grew up playing Mizuno irons, the look and feel check all the boxes.

For the first time since I’ve been doing this job, it seems like Mizuno finally has the iron that fits my game.

Price, Specs and Availability

Mizuno Pro S-4 Custom Irons

The Mizuno Pro S-4 has been engineered for accomplished golfers seeking the unmistakable feel of a forged iron with greater forgiveness and confidence throughout the set. A redesigned curved muscle concentrates mass lower in the head, while progressive offset, a wider sole and deeper center of gravity promote higher launch, increased stability and improved forgiveness without compromising the precision expected from Mizuno Pro. Forged from a single billet of 1025E Mild Carbon Steel using Mizuno’s patented Grain Flow Forged HD process, the S-4 delivers the exceptional feel, feedback and workability synonymous with Mizuno’s forged heritage

View Product

Mizuno’s new Pro S-4 irons are available for pre-order starting Sept. 3 and will arrive at retail locations on Sept. 17.

The irons cost $215 per club.

Stock lofts are as follows, but can be adjusted through custom:

(3i) 21, 24, 27, 30, (7i) 34, 38, 42, (PW) 46

Want to find the best irons for your game in 2026? Find a club-fitting location near you at True Spec Golf.

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