Howdy, as per rule 6: Tools & Engines: Use any tools or engines that get your creativity flowing—whatever works for you!
So you can use Scratch or anything else you feel like.
For your second question, i dont believe there are rules against making it cost money, but you have to ask yourself, is anyone going to pay for a game made in 4 days. Then along with that, you will perform very poorly in a jam if people have to pay to even give you a rating, most people wont. What I would suggest is work on a free demo version for the jam, then afterwards youve gained some interest then try to create a version to sell that is feature complete.