Goodness, the manual for that board really hurts my brain.
You have 2 full size PCI-E slots (the other slots that are only a length of 1 aren't big enough for M.2 to PCI-E adapters, I don't think).
The amount of lanes available depends on the CPU so I will assume you do not have an APU and you do have a graphics card.
The first full size PCI-E slot is always fixed at 16 lanes.
The second full size PCI-E slot is always fixed at 4 lanes.
M.2 slot 1 appears to share lanes with the second full size PCI-E slot, so if it has something in it, then the second slot full-size slot will be disabled (hence, you can't use an adapter in there).
M.2 slot 1 will support a PCI-Express M.2 drive at PCI-E 3.0 speed with 4 lanes.
M.2 slot 2 will support a PCI-Express M.2 drive at PCI-E 3.0 speed with 2 lanes.
If SATA port 3 or 4 are used, M.2 slot 2 is disabled and the same in reverse. It appears that it doesn't matter if the M.2 drive is SATA or PCI-E based, you still lose the SATA ports, but I can't seem to confirm that anywhere in the manual.
In summary:
Is it possible to use a M.2 to PCI-E adapter? - No, because I assume you have an M.2 drive in M.2 slot 1 and so you have no lanes available for an M.2 to PCI-E adapter in the only PCI-Express slot which is physically large enough to support one.
Is it possible to use a M.2 drive in M.2 slot 2? - Yes, providing your SATA hard drive is not connected to SATA port 3 or 4.
Please note that B450M Pro 4 and B450 PRO 4 are different boards.
