Put a spring on the shock put the bypass next to it and see how tight the fitment is. You didnt mention what the OD of the spring is or what size bypass you are running. If for instance you are running a 3" bypass and a 2.5 coilover then I dont think 4" will work.
I reccomend getting the shocks, springs, and mocking it all up. 4" will be very close. Be sure to have the bypass tubes placed away from the spring area (im sure you have all ready considered this). Also, Im no cage expert, and I know its just a start, but I reccomend a second opinion.
Sketching out concepts is a useful exercise but I wouldn't get too tied down by any dimensions unless you have the parts in your hand. As mentioned above, outside spring diameter will be different given a specified rating as well as "clocking" of the bypass tubes all affect allowable distance. You'd be surprised by simply clocking the tubes how much available space appears. Of course you could make it crazy wide but that would defeat the purpose of making a tight and compact design. I'd get everything that you need first, then worry about how it all goes together when "you" can check the tolerances rather than relying on what someone else had done.