You can use NAT and PAT to have an internet access in those systems, even you can use ISA server or Untangle.
You would need, in addition to what else you have provided you have, a PAT supporting firewall. What is PAT? It stands for Port Address Translation. Very similar to NAT, it makes the translation based not only on addresses, but also on port numbers. This allows a many-to-one relationship between the internal users and the four public IPs.
This is really simple: Get a router and/or some switches. ONE public IP can be used on the router interface connected to the ISP, whereas another router interface can be wired into an internal switch or switches, which in turn connect as many devices as you need.
The IP range you’re using on the “internal network” can’t be routed over the internet (per RFC 1918), so the router – whose JOB is to move traffic from one network to another – will allow all of your connected PC’s to flow in/out of the ONE PUBLIC IP (using the magic of NAT/PAT as mentioned above)…