I was going to use an old piece of
1x12 for the sled and probably about
4' long (8' is a little long for me).
Do you think that would work OK?
As far as your jig being 4' long....it all
depends on the length of the board your going to try and put a
straight edge on.
I buy all my lumber rough cut and mill it down to size.
The lumber yard charges me too much to straightline edge
one side of the board so...
I do it myself.
If the end of the board is too far from a hold down
clamp you may get some chatter
which in turn can ding up an edge.
So anything hanging over the end of the jig just might
need a little extra touchup on a jointer afterwards.