Nicky you've not been reading the in-game literature! The 4 stages are
Runners
Stalkers
Clickers
Bloaters
an "offshoot" stage (somewhere between clickers and bloaters) are the "Shamblers" from TLOU2.
Plus there's "The Rat King" which is the giant mutated infected from the hospital basement that Abby fights, created by several infected being trapped in a small space for 20 years and merging into one.
There might be new variations in TLOU3 that don't exist yet (I'm expecting another 5 year time jump personally).