1 x SiliconDust HDHomeRun Prime HDHR3
3 x Acer Revo RL70-UR10P
Hook all four of these devices up to a wired computer network. Connect each PC to each HDTV. Assign one Tuner to each PC and you should be good to go.
It's a much higher up front cost, but over the long run, you save the cost of leasing three set top boxes.