You can also change the callerid. GV supplies an 11 digit callerid. Leave SP1 callerid unchanged with a 1 prefix. On the other trunks change the prefix to 2, 3 or 4 respectively. The following will not change anything if the callerid is blocked.
Voice Services -> SP2 Service -> X_InboundCallRoute: {(<1:2>xxxxxxxxxx):ph},{ph}
Voice Services -> SP3 Service -> X_InboundCallRoute: {(<1:3>xxxxxxxxxx):ph},{ph}
Voice Services -> SP4 Service -> X_InboundCallRoute: {(<1:4>xxxxxxxxxx):ph},{ph}