I've asked this to the folks at BT in the past and according to Mike/Donna the 2250 will give you about 170W to each speaker with four 4-ohm speakers in parallel and the 4180 will you give you about 120W to each speaker with eight 4-ohm speakers in parallel. Same ratio goes for the 2180, about 120W to four 4-ohm speakers in parallel.
The nice part about the BT amps is the wiring is easy. Just wire each set of speakers straight to the amp and the paralleling takes place internal to the amp.
Freedom isn't free, remember to thank a vet.
2018 CVO RG lava orange, 117 Stage 3, 14" F47 and a few chrome extras. Boom crap gone.
2016 RG Special Cosmic Blue, 120ST with Dragos 2:1 and TTS tuner, FBI drop seat kit and JNR air ride. Stock audio long gone.