External Insert Confusion??

What you want to do is put the External Insert on the DI track 4 just like you would any FX, and then bounce track 4 to track 5 with Fast Bounce disabled, and Live Input enabled. Or put a send on track 4 to a Patch Point, and record form the patch point o Track 5.
 
The only tricky part is that when you click the Delay field in the EI, SONAR is going to send a ping/click to the amp that will have to be picked up by the mic at sufficient level that SONAR can detect the return to measure the latency. If you can get the delay measurement to work, SONAR will do the compensation for you. I've never tried to get EI to ping through a mic'd amp, but it should be possible.
 
Otherwise, just record the mic directly to track 5 without using External Insert as the previous post suggested, and temporarily add a Manual Offset to your record latency compensation to account for the distance between the amp's driver and the mic (1 sample/.3" at 44.1kHz) - assuming you already have your normal delay compensation dialed in (if not, get this free app, and do that first: https://centrance.com/downloads/ltu/).