A fewadditional thoughts:
Four legs on this table will allow for four chairs to fit better without interference,…customer should grasp this
You will need a fairly good size bridgeplate under the top. This will allow you to use a “tight” tenon and some lag bolts or screws to attach to the column and attach to the top further out for support (use screws with washers if solid top). I put a cove on the bottom of the bridge plate to reduce the profile of say a 7/8 thick bridge plate. You might even consider increasing the column at the top a little to give you more surface area, will not be seen unless you are laying on the floor.
I think you could get by with a 3” column, adding 1” is not going to affect the visual with the chairs placed around the base. The customer is going after a contemporary look, and 3" is fine for that size top.
With correct joinery, the legs can besecurely attached to a 3” column.
It’s doable, Justmy two cents.