Yes, it is feasible to combine PCIe 4 lanes into multiple PCIe 3 lanes.
Yes, it is feasible to combine PCIe 4 lanes into multiple PCIe 3 lanes.
You can divide a single PCIe 4.0 x16 interface into four PCIe 3.0 x8 segments while retaining the full PCIe 3.0 bandwidth per segment.
It's due to the way PCI-E is built. Unless you have a chip that mimics a network switch, you're limited to splitting the connections if your motherboard and BIOS allow it. This means dividing into certain "groups" of lanes. There are additional constraints, but I'm not entirely sure. I believe the maximum is around 7 groups, though you can't have all 16 PCI-E x1 slots—just combinations like x8 plus x4 plus x4 divided by 3.
The spec 4.0 allows handling more data per line, plus some extra capabilities that come with the usual features. For instance, 4.0 is roughly four times quicker than the original 1.0. 3.0 is about three times faster than 1.0 was. This trend continues similarly. As @mariushm mentioned, you could create separate groups using a switch-like system. Let's consider a raid controller for example—it can convert four lanes into eight SATA ports. Or an M.2 card can change four lanes into two M.2 slots. There are also PCIe risers that transform an x16 slot into four x16 slots. But the connected cards will still use the same 16 lanes. The exact process isn’t clear to me; it might involve switching, or the board may have a chip that splits the lanes into four per x16 slot.