If you have 4 pots lines and use one as your main line, what happens when someone calls in and then someone else calls in? Does the second person get a busy tone?? Does it transfer the call to one of the other lines if the first caller punches in an extension so it frees up the main number for the next caller?
if the second caller calls into the same main line, then it will get a busy signal, since this has nothing to do with Asterisk but with your PSTN provider.
The telco who supplies the four POTS lines probably offers hunting service. If you have them arrange the four POTS lines in a hunt group, then calls dialed to the main number will arrive on the first non-busy line in the group. This is under the control of the telco central office (which decides how to connect each inbound call) and not under the control of the customer equipment (e.g. your Asterisk machine).
Some telcos charge extra for hunt service; some only offer it on business service.