Is there an answering machine-with 4 line capability-that will work on an old Comdial Phone System?

You can go down the any store that carries answer machines and pick up a couple of two line machines, but you will have to connect them directly to the lines you want to answer and also have the lines go to the system. So in other words you will have to split the lines with the system and the answer machines.
Hope this helps
dtmf

If you look hard enough, you can also find a device that will let you use 1 phone on 2 lines. This will turn a single line answering machine into a 2 line (only one line works at a time but it would cover both lines) I suppose it would be possible to make a 4 line with 3 of them (plug two into one and then you would have one output for the answering machine, and 4 inputs for lines) They are sold at radio shack for about $20. Be careful not to go the other way (two phones on 1 line) which is the more common found device