I purchased the TRBX304 2 months ago and after several weeks decided that I should have gotten the TRBX504 after several discussions with @howard . I did not like the tone on the TRBX304 ie. it was too punchy for my liking, probably because of the pickups, circuitry, and the fact it is always in active mode.
I thought about replacing the ceramic pickups with the alnico, that the TRBX504 comes with, and installing an active/passive switch, and I believe the active circuitry is also different but for the cost of doing all this it was just easier to purchase a TRBX504, which I did.
I am more than happy with my TRBX504 and the price difference is more than worth it.
I leave the active/passive switch in the passive mode most of the time because I like the more mellow tones that it produces.
The link that @Mike_NL Mike gave is handy to show all the differences.