Character Select 3 was almost considered retconned due to the interest of a forum game not being as strong. Thus 4 was considered a reboot mostly using Characters from 1 and 2 for returning characters
Character Select 4 was one of the longest in production as the future seemed unsure of how a forum game could live on. However due to the art portal being released this was an oppurtunity to drum up more interest. It started in 2007 with a early build in 2009. It was not fully completed till 2013.
Character Select 4 was the first to have several characters cut. Wario, Thwomp, and Samus were initially planned for the roster to return. In addition Obama, Parappa, Google Logo, The Nostalgia Critic, Bert and Ernie, The Sun, and an Outlet were planned newcomers. Some of which made it into later entries.
Donkey, Captain Commando, Butters, The Boyfriend, were considered for 8. Donkey was sceapped due to roster constraint, Captain Commando due to an over abbundance of Capcom reps Butters and The Boyfriend were thought to be better off as aome of the first newcomers to 9 rather than DLC
Some characters are fan requested. Jibinyan, Doki and Nabi, Skibbidi Toilet, and Marko are some examples.