Too many choices, not sure which direction to go

I am having a new central heat and ac system put in. My outside unit is a 2.5 ton but needs to be a 4 ton. I have had at least 4 quotes and the last guy is a small operation, him and a a helper and he said he could do it for $4300. This includes a new return in my ceiling, 4 ton Gibson 14 sir unit outside and a new furnace. This was my cheapest quote, some other places in town go up to 10k but that is replacing all new duct. The lower prices are using my duct but making some adjustments. Any things to ask or look for would be appreciated.