I decided to run into Mariposa all guns blazing, after clearing up the vaults water problem, I've killed everyone I can reach without passing a yellow forcefield and I can't actually find a way past them, specifically the one blocking the elevator to level 4. I found a terminal that said it lowered some forcefields, but I'm not seeing any difference, all the yellow and red forcefields appear to be still up.

This is Fallout 1 fully patched running on Vista SP1.

I remember this part, took me the whole day to figure it out. I think use the Repair Button on the computer/monitor next to the force field. Its been a while, so I might be wrong.

Yellows can be disabled by using the repair skill on them or by blowing them up with explosives.

Reds can not be disabled. You just have to walk through them and take the damage.

What you gotta do is:

1. Find a computer (not Mach IV) in floor one. Use Mutant radiophone on it. It will give the forcefield commands to the radio.
2. If i remember correctly, use the radiophone in a red control panel near forcefield, OR, simply use the radiophone in inventory. I think it's the latter that works but don't remember correctly.

It disables all but level 4 forcefields. In lv4 there is one field, which you can destroy with bomb.

SPOILER: