said by BiggA:NO NO NO! 4" is NOT OK. Cable should be buried at least 18-24" below, in a trench with crushed stone and PVC conduit, and any appropriate barriers/drainage material put in. Direct burial is just doing a crappy job. Ours are probably 24-36" down in ~2" PVC conduit that is laid in crushed stone. It runs along with telco and power lines, each in separate conduits. Even our pool wiring is probably 18" underground, in trenched-in conduit.
Well then that would require the customers paying to have it put that deep. Up here in Canada there is limited time to bury cable and 9.9 times out of 10 drops are just spaded in anywhere from 4-8 inches depending on the shovel and ground levels. I agree everything should be in conduit but then expenses will go up and so will your bills. I have been doing this for many years and the time I was in West Palm beach working for adelphia as an installation tech we had to bury all new drops, we had 25 techs, I can't see 25 trenchers, rolls of conduit going out with every truck to bury a line 25-50 ft from the tap, imagine the costs involved.....its not happening buddy!