This is an old question by now, but I wonder if a plausible explanation can be expounded based on Palpatine's mastery of the ability known variously as "Force stealth", "Force Concealment", and "Buried Presence"?
It is universally accepted that Palpatine was one of the most formidable Force users in the entire canon and expanded universe. He even prevailed against Yoda, a centuries-old Jedi and one of the most gifted masters in the Order's long history. To conceal his presence in the Force would have been a monumental task, and that Palpatine managed to hide his dark nature even from wise masters in the same room for over thirty years is testament to the Sith Lord's ability.
I hypothesise that when Palpatine attacked the Jedi masters who had arrived to arrest him, he also stopped concealing his presence in the Force. The effect of this would be overwhelming to the Jedi for all the reasons posited above: they struggled to believe that the Sith were actually still around, they couldn't fathom that Palpatine was the Sith Lord. Suddenly, they were faced with Palpatine's overwhelming dark side presence, perhaps even strong enough to temporarily disrupt their own affinity with the Force.
Stunned and bereft of their supernatural reflexes and instincts, the Jedi masters couldn't even raise their lightsabers to defend themselves against the Sith Lord's blitz. Only Kit Fisto and Mace Windu, both possessed of notable Force affinity, survived the first few moments to even begin fighting. But Palpatine's gambit had paid off; he had been able to use the element of surprise by negating the Jedi's Force-enhanced reflexes and limited precognition in order to mostly even the odds. It was now two against one instead of four against one. Maybe Fisto remained somewhat disoriented for a few more moments, so he was easily cut down. Maybe even Windu was also disoriented; as the duel spilled out into the corridor, it still seemed that Windu was getting his bearings, and was nearly stabbed as well.
Again, this is simply my headcannon--grounded loosely on what we know of Palpatine's abilities and the nature of the Force. It was the only way I could think of that would explain why four of the Order's best swordmasters were so quickly dispatched... and not even with what appeared to be much technique from Palpatine. I mean, I was really rather disappointed by how the whole thing went down until I started justifying the events in my head this way. This would be the only mitigating factor I can conceive that would redeem the whole event.