Trying to work out how to do this:

We have an ADSL 2+ connection into Ether 1 with a 1TB data limit
We want to split this 1TB data limit across 4 LAN's with the following limits:
eg: Ether 2 - 300MB, Ether 3 - 300MB, Ether 4 - 300MB, Ether 5 - 100MB

These data limits are monthly and will reset at the end of each month

Can someone please point me in the right direction to setup this.

What would happen if an ethernet interface reaches the limit ? You want to stop connection or lower the bandwidth or ... ?
Are all clients on the same subnet, or in 4 differents subnets?

Simply make a hotspot on the desired interfaces.

Assign them usernames and password with desired value of total bytes out

They would eventually disconnect after crossing the data limit.

also at the month end just hit Reset All Counters in Hotspot menu and you're good to go for another month.

Thanks for the responses. is there s way to automatically reset the counters? this isn't something we will be managing proactively for the client so if it can be set to reset after 30 days that would be great.

this isn't something we will be managing proactively for the client

You wish!
If you can do it from the CLI, you can script it. If you can script it, you can schedule it. If you can schedule it, the router can do it itself.