4 IP public over 1 conexion ethernet

hello guys, to see if you know this.

I have my normal wisp two lan wireless cards and arriving at a switch for customers who come via UTP cable.
internet for output to other lan card that connects to an ADSL modem and finally to the Internet.

well my question is this, my internet provider gives me 4 public IP but my server mikrotik takes one and when they go to downloading from rapidshare, megaupload, etc.. tells me my IP is already in use and that I expect but the truth is not so, is another user in my network that has occupied the connection.

I wonder if there is any rule to solve this problem in order to have the public IP 4 actual handing me my probeedor through my ADSL modem.
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Post the relevant configuration sections please.

It’s easy enough to set up NAT so you can reserve one of those IP addresses for your connection and everyone else can use the other three.