hey, i was always wondering, i see guys in music videos playing drop C and lower with a 4 string bass. Now I own a 4 string and I jam out in drop c all the time, and it still sounds good. But if I go any lower, it starts to sound muddy, and im wondering in a band like slipknot their bassist uses a 4 string bass and they play like in drop A or something lol. How dose this work??? dose he like stick a b string in there instead of an e or what?
^Thats not what he's asking/saying.

A lot of 4 string bassist go BEAD, or just have thicker strings (either all 4 of em or just lowest).

You can do this easily by just buying a set of 5 strings, then using lowest 4, for example. All you'd have to do is re-set up your bass. its not NECESSARY, but HIGHLY recommended.
wouldnt u have to like adjust ur nut though? like make the gaps bigger to fit the larger strings in each one?
You Might. You can just file it down tho, like with a nail file or something. A electric bass guitar nut is Very inexpensive to replace compared to other things you could break while setting up a bass. (ie. A bridge (hard to **** up tho), an UPRIGHT Bass bridge/nut, necks, etc.)
ok sooo if i got it customized like that, i would beable to play in a regular tuning "EADG" or dropped D even? Someone told me, if i did that i would f*ck up my neck beacuse of the tension with the strings
Thats one of the things you'd need to adjust.

With time, and some hands on experience, your able to set you're own instrument up. I'm lucky that behind my dad's back i try out things on my Squier 4 string From my own experiences and a bit of online guidance, i can now set up my own action, intonation (both upright and electric bass) and change strings and redo wiring (in case somethings gone shitty, re-soddering (SP??) )

I Cant, however, yet, set up my truss rod/neck. I just havn't tried, because thats one of the few things on an electric bass that if you mess with wrongly it could cost you. A broken nut is nothing, a broken bridge is a pretty big deal, but **** your neck and you're screwed. It's not that i personally DISCOURAGE doing it, a lot of people argue one way or the ohter (doing it by urself), vets and nubs alike on either side, its just that i myself wouldnt do it. Or atleast not until i NEED a neck adjustment.. i might decide otherwise. but for now i dont even need to.

So you decide for yourself if you want to do it. All it really is is Gradually turning a Rod in your neck to loosen/tighten it according to the new set up you're going to have.