Do Apple Lossless tracks remain lossless when sent via Airplay to a 4th gen Apple TV?
I'm assuming you're talking about AirPlay Screen Mirroring from a Mac to an Apple TV. In that case, yes, you are loosing signal quality from compression-before-transmission and decompression-after-reception.

In particular, the audio part of the AirPlay Screen Mirroring is compressed using the AAC-ELD codec to ensure low-latency sound encoding. However, there is a signal loss involved.