It already ships with .net4. You need to update whatever package your installing. ie, the old Intel Rapid Storage Tech software would report needing .net 4 but the newer versions were fixed...
To use or not, open control panel, programs and features, left hand side at top, click on turn windows features on/off. Make your selections. Try and run the programs in compatibility mode, try Win 8 first ........
4.6.2 should be backwards compatible with 4. I've never heard of a program that couldn't run net 4 progs on 4.6.x Which program are you having trouble with?
Murphy's right, all versions higher than 4.0 are in-place upgrades, which means they are binary compatible. You can tell from the fact that the assembly version number (as part of the assembly name, not to be confused with file or product version) is still at 4.0.0.0. They include new APIs but don't change existing ones, and they're still running on the same CLR. I'm also curious what application it is.