If there is a way to do this with the signaling please enlighten me.
Your provider (KPN ?) should be able to give you Point to Point ISDN, with four 'Direct' numbers instead of what you have now ( Point to multipoint ISDN ). But, maybe you have other equipment that has to remain working as it is now. If you ask the provider to make the change, other equipment will stop working.
Would it be a viable alternative to split my line and stick a second ISDN card in my server, thus having the ability to assign 4 MSN's. Although i wonder if this would properly work when the 2 channels would be in use and a third call would be initiated... More expensive than a software solution but if its a way around my problem
I don't think it can work as you described, but maybe you can manually simulate the fact that 'four lines' are present. That's the idea behind the 'database' change I suggested before.
How to proceed ( ! just an untested idea !)
1) Setup a TEST running system, based on two MSN's you have.
2) Edit ( using for instance mc in the console ) the 'selintra-work' database. Databases are located in /home/e-smith/db/
Goal is to add a modified copy of the ISDN related 'lineIO' entries, adding the 2 missing MSN's. ( Also increase index 'ZAPx-y')
3) When done copy 'selintra-work' onto 'selintra' database. ( Not a good way to go, but ... )
4) Do a 'commit' on web global web panel ( ... needed for re-generating extension.conf )
5) Check if you get the good settings in 'trunklines' panel & try ...

Best,
hervé