I think there is overuse of reverb throughout creating a low-mid hum throughout -- perhaps roll off some of that as you incorporate more instruments.
Mix at 1:17 is sounding great on your bass, but I think the cello could be thinned out a bit and made into an ensemble at the same volume. I think it actually needs a wetter, crispier sound.
Structure wise, going great into 2 min. Liking that drop. Wanting actually more low mid and high mid here, and a bit drier tint here, especially at 2:15.
At 2:35, I think the low piano chords could be thinned out note wise or replaced with saws. Also, the attacks on each note all sounding at the same time is a bit grating for that naked section. A natural hand roll as it would be played might sound better.
3:42 is way over compressed, more than 3:24 even. It is losing the natural character of the synths and the transition sweep is getting lost as soda fizz on top of any reverbs there. The side chain is also almost totally lost in terms of actual contrast it is providing.
At 3:59, same issue as 2:35. Changing the velocity to be softer here, as it's the outro, may help.
Otherwise, I think you've done a fabulous job at sound design and made a good composition. Great work!
Question, does Nexus 4 still have the keys from the original Nexus? That piano sounds so familiar!