you need a ChannelPlus 5545. 4 analog channls in, one cable TV connector with 4 channels out. they need a blank channel in between, so 21 - 23 -25 -27 workx. Less than $200 new, ~$100 used on eBay, mine have worked for 10 or 15 years now.
I put my outdoor 5545 on one leg of a cable TV splitter, my antenna on the other leg, using the splitter as a combiner and using the input as an output. works fine.
Edit: Look at the ChannelVision E4200IR:
http://channelvision.com/product/4-input-rf-modulator/
same idea, with a digital readout indicating channel number