Assuming of course that appropriate splitters are used. Also, STBs 2-4 might not be added immediately so the outputs would be terminated properly. I guess my main concern is the 50' coax run.
50' is not a problem. I had runs over 75' (I upgraded to Cat). The biggest killers are using quality splitters with the correct bandpass, coax terminations, and caps on unused splitter ports to reduce standing waves. I'm assuming rg-6, but even with older cable (Rg-59) that run should be fine.
If you are adding 2-4 later, then eliminate the first splitter until you do. If that splitter is verizon supplied, move it to stb1/AT and then use a barrel connector at that original point until such time you add additional STB. That will give you time and keep your signal stronger. Will it really matter, no but if you are particular, yes. If you do get one, get a hi-freq one (3GHz).