Well in the season 5 premiere 'The Unstuck Man' a scientist by the name of Dr. Gieger is working on merging all universes into one and he runs an experiment with Quinn's double who is played by Robert Floyd. He sends his Quinn into our vortex with an invention of his, and the two Quinns collide. This collision merges the two together leaving their Quinn as the one who they look like. Because he is not our Quinn, to Remmy and Maggie he is just a Mallory, so that is what he is being called instead of Quinn.
Because of this collision, Colin loses his anchor to whatever universe he is on. What this means is that he cannot stay on one earth on his own because he lost this anchor. He is what they call 'unstuck'. You soon find out that Dr. Gieger is also unstuck.
The other new slider, Diana Davis, is played by Tembi Locke. She worked with Gieger and Quinn and now has joined the group so she can try to seperate the two Quinn's and also get Colin his anchor. Diana has brought with her a devise called a 'Paddle' or PDL. Kieth Damron, who wrote most of season 5, explains this as sort of a Star Treck Tricorder, but used for Sliders. It is scientific equipment for multi-dimensional use. See the 'Year Five Journal' for the episode 'New Gods For Old' and he explains it more.
Since the second episode, Dr. Gieger has not been mentioned or heard from.
The same goes for Colin.
'Strangers And Comrades' wraps up the story arc that deals with the Kromagg superweapon that was the driving force of season 4.
Since the episode 'New Gods For Old', our Quinn may be lost due to what happened to Mallory with this group.