Cin never actually fought against Grievous. We know from ROTS that Grievous was no push-over and was giving a good fight to Obi-Wan (who I think is better then Cin).
Grievous didn't do that good against Kenobi. The LOE quote leads many people to believe that Cin > EU Grievous.
And those 4 knights are not among big names like Anakin, Obi-Wan, Mace and Yoda in PT period in terms of skills and capabilities. When Grievous met one of the best in true sense (namely Obi-Wan), it was game over for him.
Still, taking out 4 Jedi who were strong enough to be given the task of protecting Palpatine is something that QGJ couldn't do. And yeah, he lsot to Kenobi, so what, Kenobi > Maul.
TPM Qui-Gon was no joke either. Obi-Wan said this about Qui-Gon: "Qui-Gon Jinn was perhaps the greatest Jedi alive, a commanding presence at Council, a strong and brave warrior who refused to be intimidated by even the most daunting challenge, and a good and kind man."
That was Kenobi, obviously he'd be biased towards his own master. Plus it only means that QGJ is strong, it doesn't mean that Jedi like Cin, Yoda and Mace couldn't take him.
He was indeed defeated by Darth Maul but Maul was no joke as well as Sidious indicated in TPM about his skills by saying "You have been well trained, my young apprentice. They will be no match for you." Sidious indeed was confident about Maul's capabilities.
Yes, Maul was good, but not better than EU Grievous, who a lot of ppl think Cin could take due to that one LOE quote (myself included).