Hi Muralman1,
The Flying Moles are optimized for 4 to 8-ohm operation, so as long as your friend's 4-ohm speaker doesn't have a broad dip down to 2 or so ohms, then they should work fine. Apparently they can be biased at the factory to work more efficiently into very low impedance loads. And you have to consider the speaker's efficiency, the room size, and how loudly they're going to be played, as well...
The Flying Moles are optimized for 4 to 8-ohm operation, so as long as your friend's 4-ohm speaker doesn't have a broad dip down to 2 or so ohms, then they should work fine. Apparently they can be biased at the factory to work more efficiently into very low impedance loads. And you have to consider the speaker's efficiency, the room size, and how loudly they're going to be played, as well...