4. Voltage controlled filter, (diode ladder VCF), as used in the Roland TB303

Diode ladder, but also in various Sallen-Key designs like the Steiner-Parker Synthacon which we all now know from the Arturia Minibrute (Yves Usson probably made more of these filters than Nyle Steiner ever did!) and as I've mentioned elsewhere the Korg MS50. I think the Yamaha GX1 filters used a diode bridge too, probably using discrete transistors similar to the Korg 700S filter.