To unlock that fourth core is usually just a setting in the BIOS/UEFI. On my little Asus M4A88TD-M that's how simple it is anyway.
I don't see a huge downside to unlocking a lower quality core most of the time. I have a 3.2Ghz Athlon II X3 450 Rana that it unlocked the core and still serves and does an occasional game just fine. What it can't do any more is overclock very well. I can get 3 cores up to the 3850 area, but with 4 cores it feels pretty limited to 3.4. All it does at that point is fall on it's face and pretty much instantly reboot.
IMO, I'd rather run around with faster yet fewer cores than be limited by speed of a limping 4th core. If you are bellying up to the computer for a few hours of editing media or running some long render that will utilize that core better, then just reboot, lower clocks, enable core, then boot up. Profiles really smooth this process out. I have 3 profiles which are stock, 3.8x3, & 3.4x4. Takes just about 1m to do the whole process and shift modes.