I agree 100%. At the very least, there should be some functionality that prevents a GCU member from buying a 4th copy instead of placing the responsibility solely on the member to remember the number of purchased copies.
It's in the online GCU FAQ, if I'm not mistaken.
Its in the fine print. I too was unaware of this if it wasnt for my CAG family talking about it earlier in the year.
They need to make it more obvious or put some programming in the computer that prevents you from buying a 4th copy, like you suggested. I can't find it in their fine print, but I did find it in their forums:
Q: Once I am a member of Gamers Club Unlocked, is there a limit to how many new video games I can buy with my 20% discount?
A: Yes. Members are limited to 3 copies of any one title (example: 2 copies for PS4, 1 copy for Xbox 360 = 3 total).