4 weeks without food is perfectly fine for the majority of fish. I've gone 3 months without feeding before. And their is a member on here who said he went on a 3 month vacation without any waterchanges in between and his fish were fine, temporary sky-high nitrates is ok as long as it is temporary and temporary alone. Just give them 1 colossal feeding when you get home followed by 2 90% water changes. That should do fine.
I speek from experience alone as I've been through exactly what you describe and the fish were doing great when I got home. Little skinny, which is actually very healthy for fish.