i am a guitarist who wants to start playing bass because i've been told and i realised that it suits my playing style and talent better
my friend told me i should start out on a 5 string because i'm a big fan of downtuned/extended range metal (i.e. korn, mnemic, fear factory, deftones, suicide silence, whitechapel, machine head. etc)

i just wanted to ask, does it matter if i start out on a 5 or 4 string?
In a word. Nope.

5 and 6 string do have their challenges but then switching from guitar to bass does as well. A 5 or 6 string gives you efficiency of movement.
It doesn't matter what you start on when it comes to learning. But considering your musical preferences, go for the 5-string. I think that if you'd buy a 4-string now, you'd want to buy a 5-string later so why not go for that from the start? With a 4-string you'll have to tune so low that it'll just be one big mess in terms of sound as well as playability.
If you were just starting bass without being a guitarist already then I'd say go for a 4 string, but since I doubt you'll be going through beginner sort of lessons that assume you only have 4 I don't suppose it will matter
With the 5 string you wont have to tune down to play along with anything higher than b standard, big advantage.
playing a 5 and playing a 4 are the exact same. the nek might be a litle thicker, but thats not really an issue for most people. the techniques are all the same, everything is the same.
no sir away a papaya war is on
I started on 5string and the only difference i see between and other bassists i meet is that i highly prefer the feel of a 5string. go for it!
dean edge one 5 string
Schecter studio-4
Samick fairlane-6
Ibanez sb900
Ibanez btb775
Fender p bass special deluxe

Dean Del Sol
Ibanez prestige rg2610

Peavey TKO 65
Peavey vb-2
Quote by the_perdestrian
listen to revelation, for he is wise in the way of bass-fu
It makes no difference learning on a 5er than a 4er.
Damn it! Disable can't use disable to disable Disable's disable because disable's disable has already been disabled by Disable's disable!
Well all this depends. If you just want the low notes, just slap on some heavy gauge strings and tune low. But if you want high notes as well, a 5 string is your best bet. Plus you can still play standard tuned songs.
Ibanez SRX-365
MIM Fender Jazz Bass
Yamaha B50-115ii
Boss CS-3
If you can play a 5, you can play a 4. Most people have trouble switching from a 4 to a 5, and vice versa. There's really not much pro or con to it.
1989 Kramer Showster
Rogue RADH Acoustic
Fender CD-60 Acoustic
Washburn XB-100 4 string
Peavey Millenium AC BXP
Fender Rumble 150
Crate BV60H Head
Crate BV412 Cab
Vox AD15VT
ThrashMetal014 is right, there's no pros or cons to learning on a 5 or 4 string bass. I mean, if you're going to be playing music that requires you to down tune a lot, you might as well get the 5 string, so that's a pro for you personally, but in general it makes no difference.

Kind of off topic: Any 5 string players find going back to a 4 string both really weird and really fun? When testing an amp in a store recently I played a 4 string for the first time in ages and my hands just flew over that thing, it was great.
There's absolutely nothing harder about playing a 5 string than a 4 string, it's just another string like the others, and if you find out you don't really need the extra string, you just won't play it, simple as that.

I started out on a 5-string that my dad had lying around from one of his friends, but I never actually used the B string, it still went very well, and I found the extra string actually helped me as something to rest my thumb on, making it easier for me to pluck the E string
I started on a 4 string, then a few years later after buying 2-3 4 string basses, i moved onto a 6 string bass.

it doesn't really matter, 5 string does have its advantages over a 4, and 6 string has advantages over a 5.