How much weight can a 4-thick concrete slab withstand?

Typically we like to limit the weight to about 40 lb/sqft on an unknown 4″ slab.

In most cases you can go to 80 lb/sqft in isolated spots, but unless you know roughly what the soil bearing and reinforcing is for that slab you might be looking at cracking.

Keep in mind that 40lb/sqft is sufficient for a typical passenger car.

There will be people who will give much higher values, but without knowing a number of things you aren’t likely to know you probably need to keep it at or near the 40 lb/sqft.