Is it possible to tune my 4 stringer down to lets say, B-E-A-D? And if so, does it really makes sense to do and does it sound like crap? Man i'm kicking myself for not buying a 5 string in the first place. Thanks.

If you get the right gauge strings, sure.
You'll probably need to make some neck/setup adjustments for the increased tension, as well, but it's not really that odd.

Doesn't Tim C do that in Rage Against the Machine?

BEAD is 5 half-steps under the standard, but it's possible. As it was said, just get a pack of 5 strings, and throw away the G string. Might need to adjust the trussrod and adjust the nut.

If you swap out the strings you have to file the nut for the bigger strings. Your choice. Try tuning you E string to a D that will give you a better sense of the basement tones.
On the last album I played on (metal and rock) I tuned my 6 string down a whole step to get that slamin basement bass.
The B string was a little floppy but it was what they wanted.

Doesn't Tim C do that in Rage Against the Machine?

He used standard and dropped-d tuning with rage, BEAD with audioslave...

Ok thanks everyone. I'm going to just tune down and see how it sounds. I might just have to wait for another 6 months or so and save up for a 5 string. Man i kick myself all the time for not buying a 5 string when i had the chance. Oh well. Thanks again.

it's going to sound really floppy with regular gauge strings. Trust me, i've tried. You need a 5-string set and a neck setup for it to sound any decent.

it has been done before. i think that Billy Sheehan had a double-neck with one neck tuned normal and the other tuned BEAD. ill see if i can find it right now.

Update: here is a link to a page on Billys website that talks about the bass www.billysheehan.com/gear/instruments_09.html

I only use 4 stringed basses. I have more then one so its easy for me to simply keep them tuned differently. I set the action to meet the tuning as some of the lower ones do tend to be overly floppy. I use 105's on all my basses so resetting the action is important. I also pick and choose what bass will be in what tuning by its original action in EADG. The higher the original action the lower I take it.
My usual line up is....
87 Aria Pro II SB Elite II SRZ---E A D G
06 Schecter C4XXX------Eb Ab Db Gb
94 Washburn XB400------C G C F and D G C F
87 Fender Jazz Bass Special--Db Gb B E and B Gb B E

Yes its a pain in the ass to drag these around but only at the really big shows do I bother. Most often I take three or two.