They didn't make any banked inner curves that I know of but my plan was to only have the outer banks wiith the standard flat curves on the inside. The other place where there is a 180 degree curve would get the other baked outer curve which would give the other 2 lanes a chance at the high banks. Clear as mud ? Not good at explaining but each lane would at some part of the track, have the benefit of the banks.
I can see that... not optimal of course, but it should work OK. Basically you would have a 'mirror image' track, made of two main sections of two lanes each nesting in each other so all the lanes have the same length. That is a good idea, and is probably the best we can hope for here.
I remember reading somewhere about people making 'hybrid' tracks made of both MDF and plastic track sections at the same time to take advantage of some special track features the 'factory' plastic track had, like lane changing. If you can make a hybrid track to preserve these features, how hard would it be to make a single MDF or plastic sheet fourlane bank track section say twice the size (180 degree section @ 42" radius and same degree of bank and other basic measurements as the original 2 lane) to use with Revell track for the rest of the track? I have the perfect spot in my design for this curve. 
Good luck with your track
Ken