I'm sure they'll fit a Gen 4. Glock would be crazy to change the sizing.
A new size would be a nightmare plus Glock is trying desparately to get the Gen 4 to win over the buying community
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek