You said they were willing to send Ken and a woman, though they said it wouldn't be necessary, so I would go ahead and do that. Even if Ken is great, you've got 4 pretty young kids and having babysat a family of 4 kids when I was younger, it was hard. The family would always let me bring a friend and when I did the night was much easier. Granted I was a teenager, while Ken is older, but I think you said he's much older, like in his 50s. Well, my mom is too and she gets tired watching my 4 month old when I'm in the house, I can't even imagine her watching 4 young kids all night alone. We vacationed with my 6yo cousin and 2.5 yo niece in WDW in May and my mom could only watch them alone for so long. When they were together they just got so wild! Even if your kids don't normally do this, they always tend to act differently with a sitter.
The main reason I told my story at Flounder's Reef is that I think it shows the importance of having enough, qualified people to watch your kids. Are your kids used to strange men? If not, they may not feel too comfortable with Ken even if you are. Like I said, I have a male friend who works in a day care and babysits and I trust him completely, but lots of times families would call me instead of him because their child/children felt more comfortable around new women than men. (I'm sure partially the parents did too.) Or what if one of the kids just wants mommy and spends awhile crying, how will Ken entertain/watch the other 3, especially with 2 toddlers? When it's a new sitter, it's hard to know how well they'll actually manage with your kids so to be safe, I would request another sitter to come with Ken.
After all, as much as KNO talks up Ken, they're his employer, they're biased. They want you to keep him so that it's easier for them. I'm sure he is great, but consider the source who is telling you. An impartial reference would be better, but since you don't have that to work with, I would get the 2 sitters. You definitely won't be sorry having more people there to watch your kids, but you might be if you just stay with Ken since you're still undecided. I've found with me that when I can't decide on something, it's usually cause I really don't want one option but feel bad saying no. So do what you know will make you feel better, and I think that is to hire another sitter.