I team attack with the 3 drop Spidey ("While Spider-Man is team attacking, attackers you control cannot be stunned.") into Radioactive Man ("While Radioactive Man is ready, whenever he becomes stunned, you may stun target character with cost 4 or less."). When RM gets stunned, does his effect go on the queue and resolve AFTER the team attackers lose the attacker characteristic? Or does he have to try to stun a character not in the attack?

RadioActive man's effect is a triggered effect. so it works like Sinestro: GLoK works. once the attack concludes, and there are no more attackers or defenders, RAM's effect is put on the chain, when it resolves he may target a character with a cost of 4 or less to be stunned. Spider-Man is no longer protecting any characters from being stunned since they no longer have the Team-Attacker characteristic

Alternately, Mob Mentality will protect you against both GLoK and Radioactive Man.

Watch out, Radioactive Man!