re: How to size HVAC for home addition. Is my current model big enough?

Glad you tried to do the Manual J - did you use the free loadcalc.net site to do that? I know it can be a little confusing initially but I’m sure you can follow through with it. But with that said,

1. Ideally your 4 ton should have been sized properly for the 2500 sq ft and should not have extra capacity for additional 600 sq ft. Does your HVAC short cycle frequently during the warmest days of summer? If not, you likely don’t have extra capacity.

For comparison if this helps, my house, slab on grade, single story, 1993 construction, R-13 walls, R-30 attic, NE-SW orientation, porches front and rear, and a fair amount of windows on the southern exposure, 9.5 ft avg height, is right at 2,500 sq ft conditioned area, with a 4 ton HVAC. On-line manual J says 3.5 ton should be sufficient for my house, and I believe it, but it never short cycles during the hottest days of summer at design temperatures. I know a 600 sq ft addition to my house could not be cooled by the “extra 1/2 ton capacity” of my 4 ton unit.

2. Though you need the advice of a professional, I think in the long run you’ll likely be served best with an appropriately sized mini split cooling for that new addition - assuming a manual J demonstrates that the 4-ton unit is properly sized for the current 2500 sq ft.