As others have mentioned, there is a big difference between dubs players and singles players. If you have been playing 4.0 dubs it might be better to first try play singles league rather than tournaments. With league you'd just be playing one match at a time, rather than needing to potentially play several over the course of a weekend during a tournament. If you are not in singles shape, that will be a tall order.
On the other hand, if you are in shape and have had some experience playing singles in league, then you might actually find matches in tournaments slightly easier than those in league. At least in my area, league players tend on average to be stronger than tournament players. The better players tend not to play tournaments, and when they do, they tend to play up.