4' feet is good, and a most common size with Blacksky rails. 6' feet is better, and is readily available from McMaster-Carr, and is also more versatile for use in that it will cross to HPR useage as well.
I'd lean toward six to eight feet. More is better, the only down side to a longer rail/rod is a little extra drag(negligible) lowering your peak altitude. Well, that and you have to find a way to transport the thing, an eight foot rail is a little hard to manage in some cars. I just bought an eight foot rail from McMaster Carr for $25, an adapter for it to 1/2 rod so it can go in a pad we use for rods as well as rails (another $20 plus $4 shipping from www.railbuttons.com)
I'm not working for or affiliated with either business. That said, I'm very happy with railbuttons.com's service, I ordered it and got it two days later as promised. Matt has always done me right.
My 2cents. I have been using the 2 piece 4' BSR for a long time and it has been fine for MPR and shorter HPR. I got it before the existance and compatibility of the 80/20 rails was widely known. If you are going to buy the 80/20, you should get as long as you can handle/transport 6-10'.
Ditto on Matt's service and the cost-benefit of his products!
Is there a way a 8' rail can be cut into two 4' rails? Mainly because 4' would be PERFECT for LPR and some MPR, then when you wanted extra stability, you'd use some sort of metal rod in the center hole, or brackets on the outside, to make it back into a 8' rail. You could also transport it in two sections! I imagine this would take a lot of ACCURATE drilling/tapping, but I think it would be worth it!
Along the lines of what jetra is saying, I bought (2) 6' foot sections of extruded rail from McMaster-Carr, and cut one down to 4' foot with a band saw. The 2' footer that was cut off, is used as the brace to connect the two sections together. I can have a 4'; 6'; or 10' full feet of rail. I also used a small piece of rod that slips in the center hole of the rail halves.
...not to mention, it was just as cheap to ship (2) 6' pieces as would have been to ship (1).
When I purchase my 1515 rail, I will bite the bullet and have a full 10' footer shipped if I can muster up a group buy here with our club.