John_V 

You can try something like this. It's a homemade job but works beautifully. This one is for three bikes or 1 bike and a recumbent trike (thus the offset) but you can certainly modify it to hold four bikes. This one cost me less than $25.00 and an hour of my time to build. I used #6x3/4 sheet metal screws instead of glue so that if any part cracks it would be very easy to fix or modify further, if need be. I also put some pipe insulation over the PVC to cushion the spokes.