Insurance says my 2020 RAV4 is totaled but the offer is $4,000 under what I still owe

Got rear-ended three weeks ago. The other driver was 100% at fault - police report confirmed it, their insurance accepted liability right away. I thought it would be a straightforward process. It wasn't.

Their adjuster came out, declared my car a total loss, and offered me $21,800. I still owe $25,600 on the loan. So I'm looking at a $3,800 gap that I'd have to pay out of pocket even though I did absolutely nothing wrong. I have gap insurance through my dealer but supposedly it only kicks in for certain situations and I'm not sure this qualifies. Has anyone dealt with this before? Is the actual cash value offer negotiable or is it basically take it or leave it?