I agree with everyone who says to learn both. There are some tunes in which you have to use the 4th finger to get a B on the E string. Some tunes (thinking of a three-part reel with a bugger of a 2nd part - what the blazes is the name?) go from an E down to a G pretty fast and I find them impossible to play up to speed without using my 4th finger on the A string. If you do double-stops at all, the 4th finger comes in handy. And you can't get any vibrato with an open string. Having said that, I'll also admit I hardly ever use my 4th finger on anything but the E string.