If you have any keyboard with MIDI out, you can plug it into the 190-4 and use it to control your modular system. Send the Gate signal to one of your envelopes. CV1 is pitch in the eurorack standard of 1 volt/octave, that goes to an oscillator. CV2 can be assigned to a different incoming MIDI signal, common uses would be velocity or mod wheel from your synth/keyboard controller.
Or, you can hook it up to your computer and use a software sequencer from your DAW or Reaktor.
It can do some other fancy things in the menus like setting portamento (glide), but I’d just focus on using it for bringing in Gate / Pitch for now. If you want to dig deeper, check the manual.