I thought about it some more and you might want to go with the second solution instead.
Answer buttons are okay, but they will ring also the other phones when an internal call comes in for the main phone and when a call is transferred to the main phone.
Programming the same lines that ring on the main phone to also ring on the three other phones won't have that side effect. And it won't change the button assignments. Better yet, you shouldn't need an approval since it can be done in a few minutes.
Step 1
Note the lines that are currently ringing the main phone:
Terminals & Sets > [221] > Line Access > Line Assignment
Once you get there, all you have to do is press the SCAN softkey and note the number of each line that is set to "Appear & Ring" or "Ring Only".
Step 2
Set the same lines to ring on the other phones:
Terminals & Sets > [231] > Line Access > Line Assignment > [Line]
Terminals & Sets > [232] > Line Access > Line Assignment > [Line]
Terminals & Sets > [233] > Line Access > Line Assignment > [Line]
If the line is already set to "Appear & Ring" or "Ring Only", don't change anything.
If the line is already set to "Appear Only", change it to "Appear & Ring".
If the line is already set to "Unassigned", change it to "Ring Only".