Analog Four, certainly.
It synthesizes wonderful drum sounds, ideal for sampling into DT in order to free up A4 voices.
Has song mode and sends PGM change so you can use it to give song mode (in a way) to DT, if this is a feature you desire.
Has input oscillators, analog filters, and filter overdrive that you can p-lock and run DT outputs through and then sample back into DT.
Has CV outputs which gives you some additional connectivity for all kinds of other gear.
Has performance macros for an additional control style that DT lacks.
Has direct jump for quick pattern switches that DT lacks.
As it has its own advanced sequencer, it allows you to free up DT's MIDI tracks and LFOs to be routed back into the DT (bonus LFO's). All you would need is a MIDI thru or merge box.
Additionally; you could use those MIDI LFOs to take A4 to more heavily modulated places.
FX track with P-locks for more interesting FX based sequences.