Well that depends on your viewing habits. Are all 4 TVs ever in use? If so, you may want to wait until the new 4-tuner HDHomeRun announced in January comes out (sometime this summer, no exact date yet). Or you'll need to get 2 3-tuner boxes and will need to use Tuner Salad (Windows 7 only, though there are some work arounds for Win 8 I've seen) to increase the 4-tuner limit in Windows Media Center.
The other think to keep in mind is you'll need a box at each TV, whether it's a HTPC (Zotac, Intel NUC, or actual computer) or an extender (Xbox 360) to access the Homerun.
Hope that helps!