This doesn't make much sense.
For a start the TM900 is 60fps and the 1080P derived from 4K is only ever going to be 30fps from the AX100. The two are not the same thing where motion is concerned, although not visible unless watching the original files.
Also who says the TM900 is the best 1080P camera? The better 1080P is going to be from the Sony AX100 as that is from a larger sensor with better optics designed for 4K and at almost twice the bit-rate at 1080P than the TM900 manages, plus is a like for like comparison with 60 frames per second of the TM900.
Yes we all know by now that nose up against the screen 4K down sampled to 1080P has an edge (as will 8K to 4K in a few years time), although this is advantage is completely lost when that output is re-encoded to play friendly codecs as 1080P and people sit watching from a normal viewing distance.
What people seem to forget is that 4K only produces better 1080P when it is 4K from acquisition to playback screen with the down sampling happening just before display, but if we can do that, then what we are actually watching is 4K, albeit with less resolution on the display, which often doesn't matter as computer monitors aren't big enough to really show off the full 4K resolution that much anyway.
For 1080P delivery, where the output is a 1080P encoded file for compatibility with current day media players, internet streaming or Blu-ray, the benefits are lost as we are now watching 1080P.