I would say it depends on how well you play. For a less accomplished player, they may not tell much of a difference, especially if they have trouble just making consistent contact. The better the ball striker, the more important and noticeable the shaft is to the way they hit the ball. The shaft is the “engine” of the club. If you swing all the clubs basically the same and one weighs more or has a different flex, it will react differently throughout the swing. The player will either need to adjust for that club or better yet, get it changed to match the rest of the set. From a performance perspective you would be better off using a chrome 4 iron that looks different but has a matching shaft with the rest of your set. Or buy the black one and get it reshafted.

Thanks. The stiff is brand new and reasonably cheap and was thinking about getting a ref shaft for it If I see the difference when hitting
Can buy a regular 4 iron of eBay but don’t trust the sites for fakes and negative reviews and looking for crazy prices thank for the andvice

Yes there will be a difference, how much can be answered two ways: 1) working with a Titleist fitter using trackman and hitting said 4 irons with regular and stiff shafts (the best option). 2) get the 4 iron with the stiff shaft and hit the range or take it out on the course and decide for yourself.
Generally speaking, you should have tighter dispersion, slightly lower ball flight and possibly some loss of distance. However, this is all dependent on your swing speed, launch angle, spin and other factors.