Mizuno has always been renowned for producing best in class soft forged irons. The Japanese manufacturer has recently shed the idea that their irons are only for better players and has become a market leader in players’ distance and game-improvement irons. In their current lineup, Mizuno has at least four different irons that could be categorised as players distance.
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 for some players they lacked just a touch of forgiveness.
Well, for the first time in years, Mizuno has unveiled a true better-player cavity-back iron, 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 said. “Which all sound like things that are not Mizuno Pro. But it’s just very slight tweaks in all of those things.”
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 the idea with the Signature Series. Give good ball strikers several options that preserve feel, while letting them pick and choose features they like, like sole shape, workability, and more. Voshall admitted this isn’t going to be an iron that flies off the shelf, but it also fits seamlessly into the existing Mizuno Pro lineup 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 like the rest of the Mizuno Pro line, the S-4 features the same construction fans 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. A soft copper underlay and Harmonic Impact Technology further enhance the feel. The S-4 also borrows the Triple Cut sole used on more recent Mizuno Pro irons such as the M-13 and S-3. The bevelled leading edge and increased trailing edge relief help offset the wider sole.
Our take:
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. It should suit players who want a single-piece forged feel, but more forgiveness without the additional speed of a larger iron.
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 some better ball strikers who just wanted a little more forgiveness. Mizuno solved this by reprioritising the new M-13 as a “speed cavity” and more in the players’ distance space.
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; I struggled with the S-3 on this (just not a good enough ball striker!!)
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.
I’m excited to get a full set of the new S-4 in play, as my initial testing suggests Mizuno finally has the iron that fits my game perfectly. 
Price, Specs and Availability
Mizuno’s new Pro S-4 irons are available for pre-order starting today and will arrive at retail locations on Sept 17th with an RRP of €265 per club. Stock lofts are as follows, but can be adjusted through custom: (3i) 21, (4i) 24, (5i) 27, (6i) 30, (7i) 34, (8i) 38, (9i) 42, (PW) 46.
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