By accident, I found the Patcher preset 'PJM4 (4 Track DJ Mixer)' by WiseLabs, which is also doing just that and more. PJM4 is included in the native FL Studio (starting with Version 12.3.), so you should have it already. Besides blending the audio, it also includes a simple EQ, a filter and volume trimming per instrument.
How do you set it up? The Documentation for PJM4 reads: 'PJM4 is a 4 Track DJ Mixer preset for Image Line FL Studio Patcher and is designed to fit DJ's needs for the production of either jam sessions, live sets, studio or promo mixes. Record your sets in FL Studio with any controller or any mappable mixer. Use sidechain routing for input.'
This appeared rather cryptic to me, so I wanted to try it out. The attached model .flp with instructions shows how I managed to get PJM4 working, although I have my doubts whether this is the most efficient method. If you know a more straightforward way, your feedback will be much appreciated.
INSTRUCTIONS to setting up PJM4
- Place four instruments in the channel rack, assign individual mixer tracks (e.g. track 1 through 4) and name them the same as the respective channels. Enter different MIDI notes in each of the channels using the piano roll or create a Layer setup to play all four instruments from a single MIDI keyboard.
- In the mixer, select a 5th track and name it e.g. PJM4, place a Patcher EFFECT plugin on this mixer track and within this instance of Patcher select the preset 'PJM4' in the X WiseLabs section.
- In the mixer, create a sidechain connection from each of the four instruments to the Patcher track, i.e. select the four instrument tracks using the CTRL key (Windows) on the typing keyboard, then right-click on upward arrow on the bottom of the 'PJM4' track (here track 5), and select 'Sidechain to this track only' (please note that the audio connection of the instruments to the Master track is disconnected by creating this sidechain routing).
- Next, route the sidechain connections inside the PJM4 Patcher Map: Open PJM4 and right-click on the 'From FL Studio' icon. Select Outputs --> Audio, and you will see the four instruments listed. Place a check mark next to each entry by right-clicking (so the menu will not close). Finally, connect the newly created yellow connectors at the 'From FL Studio' icon to the four 'trim' modules (trim1 through trim4) and start playing.