Confident after some initial drawings (below) I tried refining some ideas, going into more outlandish designs that could come from the puffy juvenile form I'm keeping with. I kind of like the amphibian forms - they relate to the swampy environment, they're somewhat bizarre and it's a form that could develop from the pudgy mass Four emerges from its cocoon as. They could also have some translucency to them (something I might need to practice drawing) to add that alien feel. I think I've hit the start of something that I could explore further.
These below were from a previous post today but I'm including them for context. 9-21 were I think when I started getting into my stride with this.
Additionally, for the base I have taken to examining the original television show Captain Scarlet and the Mysterons, which was broadcast in 1967. It's not 1970s but there are possible signs of the trends that would mark the 1970s within the show. It's more mature and serious than it's predecessor Thunderbirds, for one.