An antenna such as the CM2016
http://www.solidsignal.com/pview.asp...0Antennas&sku= aimed at about 112° (compass) feeding a passive 4-way splitter should give you what you're looking for. If you re-use the 8-way splitter (passive I presume), you might need to add a high input preamp such as a Winegard HDP 269.
Do not try to combine OTA signals with cable. To do so will cause you to transmit signals into the air on aircraft navigation and communication frequencies.
If you are going to keep your existing internet access on cable, you will likely need to run some additional RG-6. But I think that you may be able to isolate the cable modem, freeing up the cables to each set.