4 to 5 string.

I've been asked to join a Black Metal band bout only have a 4 string bass. Can I get away with using that by tuning down or am I better of getting a 5 string for the B. Any tips?

Cheers.

Is there a rule that states you can't play black metal on a 4 string?

While I am not one of our "actual bassist" members here o the forum( more like I have a beginners model that I use to facilitate my songwriting, I'm primarily a guitar player), yes, you can "get by" with a 4 string tuned down, provided you change your string gauges to compensate, like putting the 4 heaviest strings from a 5 string set on.  That'll work just fine......BUT....it's not going to be quite that simple. Because of the difference in the string diameters, you WILL have to do some work on the nut, because it won't be cut to accept the larger gauges, and you'll most likely need to re-intonate the bridge as well.  There's also the matter of scale length....the average 34" scale will work for B, again provided the string guage is good.  If you tune lower than that, my experience says you want a bass with a 35" or longer scale.  Drop A is about as far as my "toy" will handle at 34" with a .45-.130 set, but I'm REALLY thinking that I'd like that low string to be a .140 or .145, because the low string feels just a hair too loose for me.

Just leaving your standard 4 string set on and detuning that far isn't gonna give you decent results......

Look around and find an inexpensive 5 string.  By the time you do the modifications on your 4 string, you could have learned to play the 5.  You won't just have to adjust the nut like Exo said.  You will most likely have to drill the hole for the E string tuner out to receive the much thicker B string.  Once you make these modifications, there ain't no going back.

If you look around, you can find some decent 5 strings for $300 or less if you know where to look.  I'd try www.rondomusic.com and find something there. 

For myself, I can't imagine not ever having at least a 5 string.

With that said, why am I seriously thinking of buying a special made 3 string bass tuned D G C?  LOL

Because it is a hand crafted bass that would look awesome in a band with other hand made guitars and drums.  When I do the blues album, it is going to be a lot of cigar box guitars with 3 or 4 strings, this bass, and both drums and odd percussion sounds.

Cheers guys. Looks like I've got one I can borrow for now. Depending on the price and if I can track one down here in Australia I've been eying of this. Any one ever given it a go?
http://www.sweetwater.com/store/detail/F415FMSTBLK

I've never played an ESP bass but I know they make great guitars.