TR-09 has a stereo out - so take a stereo 1/8" cable to duel mono 1/4" and plug them into the A4 inputs.
Then assign [L]/[R] for OSC1/OSC2 respectively in a single track of a new A4 project and use this to process the incoming sound from the TR-09. You can mute everything except the drums on the TR-09 (obviously, either don’t program anything else other than drums or simply mute other voices you’ve programed by turning knobs for each voice counter clockwise.) If you do it this way, then you’ll have three other synthesis tracks on the A4 to play with.
You can reference this type of setup in the A4 manual appendix that shows examples of common uses of the A4.
Also run midi CC from A4 to the TR-09 so everything stays in sync – of course, if that is what you are interested in doing.