From a numbers standpoint, it depends on the speed of your gen4 drive; but if your drive is capable of higher transfer speeds than gen3 allows, you'll technically be bottlenecked from the gen3 slot.
However, in practice, in most everyday uses you probably won't notice any difference unless you're transferring massive amounts of data to and from your SSD regularly.
That said, if you just have a gen3 slot, you could potentially save a bit of money by going for a gen3 drive instead of a gen4 if that matters to you
