I've put a 40MM on my Gen3. For me, the stock 50MM was a bit too long, as I have the S4 and I am 181CM. For me, this feels better, but this might depend on your height and preferences
Rode a Gen3 S4 with 40mm stem and it was quicker to respond when pulling up on bars. Felt better than the stock 50mm, so I bought a 40mm for mine and will install this week.