GPO enforcement and security filtering

I have a 4 GPOs that I would like to apply to the same OU and I would like to control the access to them by adding security groups of the users I want this policy to take effect to the "security filtering" field in the scope tab.

One of the GPO's is enabled for all users, the other three have specific users. For some reason the GPO that where all users have permission is taken effect an not the others. If I enforce the three groups with specific users will this enforcement apply to everything in the OU or just the users in the security filtering? In other words, does GPO enforcement ignores the security filter or the security filter still counts even when the GPO is enforced.

You may be asking why I am not using different OUs for this, well, I can't move these users out of this OU for management reasons.

I would appreciate your help with this.

Thanks,

D. Estrada