vimsynth/modules/mixer
Sums four signals, with a level on each. Works for audio or CV.

MIXER to get this card.Down the left edge of the card, top to bottom.
in 1, WITH the space, not in1 or 1.in 2). Its level starts up, so a source patched here is heard.in 3). Its level starts up, so a source patched here is heard.in 4). Its level starts up, so a source patched here is heard.Down the right edge, in the same order.
Left to right across the face — the dials in the picture.
The full reference — every parameter curve, the patching notes and the worked examples — lives in the manual: docs.vimsynth.com/modules/mixer. In the app itself, :help MIXER opens the same page without leaving the rack.