An expensive solution to your problem
first costs $2000 but you can get it on Ebay for less:
This lets you display the same source on 4 screens. (allows you to switch between 4 sources).
The other one is a matrix solution and costs $4000, allows you to watch a different source on each screen (and control the source). Note you need 2 cat5 cables per screen (one for Audio/video and one for the HDCP signal if the source is using this).
as for PC - just get a graphics card with a DVI output, connect it with a DVI to HDMI cable. Should work fine.
The final option is sky multiroom (£200 per sky box plus £120 per year) which is considerably cheaper.