So, I'm trying to convert a hair made for the sims 4 to the sims 3. So far I've scaled the hair to fit a sims 3 head in blender. But before i move forward...
the mesh has a forehead. XD
Do all hair meshes have foreheads? Do I need to remove it from the mesh? How do I do that?

The "forehead" UV should match to a texture that gives fine hair detail in that area. I guess TS4 uses that instead of scalp and face textures found in some TS3 hairs. If positioned correctly, you probably shouldn't just remove it. Check the TS4 texture, and position the texture for the forehead part accordingly.

TS4 hairs can use a scalp to blend the head and hair together with unique hairlines.
TS3 has a separate scalp that can be textured for a similar effect, but the UV is significantly different than the TS4 one.

You have a couple of options though-
1. TS4 grabs that skin texture from the actual skin, so when you convert the hair you won't actually see that colouration anymore. It's effectively safe to just leave it there.
You can fit it snugly over the TS3 scalp and face, and if there wasn't a significant scalp part it will look just fine. It may help to convert the mesh to .obj before working on it so you can get a better idea of what it looks like as a mesh - as fresh out of S4S it has all those TS4 texture compositors still plugged into it.

2. If you're ever converting a hair that has a significant scalp texture- for example the roots of braids, babyhairs, or shaven sides that need to be preserved for the hair to look correct- the way I've seen creators convert their own meshes is to put the hair and scalp texture on the same image and fit the UV to it. Then it's fitted to cover the TS3 scalp same as above, and all parts are visible and considered part of the hair.

Thankyou! how do i fit it snugly over the scalp? Just moving the verticals by hand?

Some may fit nicely just by moving the entire mesh into place, utilizing Grab and Scale to make sure you don't see the TS3 scalp clipping through the one on your mesh. If you preview the texture and those areas look like they are floating too high above where they should be sitting, then you can go in and move the relevant vertices into place.

I have a hair tutorial that starts here and contains some of the Blender shortcuts you might find useful. You don't necessarily have to follow the entire tutorial from that point, once you're happy with the mesh you can use it with whatever steps and tools you prefer

Gah! Awesome, thank you so much! This will be very useful!