What happens if one puts a PCI-e x4 card into an x16 slot that's only connected to 4 PCI-e lanes?

[FrostigerFinger]

Can the card use ALL of the bandwidth available to the slot, or is that only possible for a full x16 card?

I know that a x16 card in an x16 slot @ x4 speed is going to get the full x4 speed,
but is the same true for an x4 card in an x16 slot @ x4 speed?

I'm thinking about using a PCI-e x4 m2 to PCI-e adapter in the second PCI-e x16 slot (only 4 lanes for this one) on my old mainboard, but I won't do it if I do not get to use all 4 lanes.

 

[crashtech]

It will work at x4, that's the way the slots are designed, with the x4 data connectors closest to the I/O shield.

https://en.wikipedia.org/wiki/PCI_Express#Pinout

 

[FrostigerFinger]

It will work at x4, that's the way the slots are designed, with the x4 data connectors closest to the I/O shield.

That's what I thought the smartest solution would be, thank you very much!