100-300k seems a little high using that color level. Are you just noting the total amount of bandwidth used by the server and dividing by four? You might want to account for other traffic from the box.
I'm assuming you're monitoring the switch port or the server NIC with PRTG? Do this: Monitor the traffic with no users. Write that number down. Add one user. Write that number down. Repeat. Find out what the increments are. This number will be inaccurate (in fact, very misleading) if the users' applications within the session generate LAN traffic : for example, if they're running a SQL client that connects to a local SQL server, or a local file share, that's not internet-bound traffic
You could measure the bandwidth used by ICA (TCP 1494) at the gateway if your router supports that kind of monitoring. That gives you an immediate and correct answer for total bandwidth.
What problem are you trying to solve? If you need to minimize bandwidth, Citrix has a lot of options for defining policies, as well as optimizing different parts of the ICA protocol. What version of Citrix are you running? If you're running a recent version at the highest-level edition, it has SNMP support for monitoring, the answer may be right in there and you could monitor that with PRTG.