It was just a way to quickly get the duel between Mace and Sidious, I don't think Lucas wanted to pro-long things with 4 Jedi. It could've been handled better, maybe having Sidious a bit faster on the attack - feels like it is in slow motion at times.
Someone pointed out either on these forums, or theforce.net forums that Lucas likes to have one on one duels. Notice how Obi and Qui-Gon are quickly seperated in TPM, Dooku and Obi-Wan, then Dooku and Anakin in AotC, the duel at the beginning of RotS when they are trying to rescue the Chancellor, Grevious quickly looses two of his hands. I think Lucas likes to get it straight to the point without too much confusion for the viewers.
Another thing that may have contributed to Sidious' attack was the fact that they were in cramped quarters, not a whole lot of room to maneuver. I am also not sure if they realized just how powerful Sidious was.