Malgus kills Vader in a matter of few seconds, during which Sidious takes on Dooku, Maul, Savage and Asajj. I guess Dooku and Maul are good enough to stop Sidious from instantly killing Savage and Asajj, although Palpatine will still obviously hold the upper hand. And Savage+Asajj are kinda tricky, they require Maul and Dooku as superior combatants to hold Palpatine from slaughtering them, but are good enough to be pretty useful against Palpatine, especially since it's 4v1.
I guess that while Palpatine probably kills Asajj or Savage when Malgus murders Vader, Palpatine will soon be forced to face Malgus+Maul+Dooku, in which case Palpatine goes down.