From the Super Mario Wiki
World 4-Rainbow is the rainbow level of World 4 of New Super Mario Bros. 2. To access this level, the timer must end with the two digits "44" (that is, "x44", where "x" can be any number) when the player reaches the Goal Pole of any World 4 level. Once the level is completed, it can be accessed again by satisfying the above condition once more.
Mario begins on icy ground under several ice blocks and in front of a Pipe Cannon. Before entering the pipe, Mario can jump on the platform to get some coins. Entering the pipe shoots Mario to the sky where the Coin Heaven awaits. This level is slightly different from the earlier Rainbow levels as Mario will be walking instead of riding on a moving platform. The ground is mainly made up of various colored pipes and also rainbow colored platforms can be found. When Mario first touch the ground, the screen begins to autoscroll and coins drop in diagonal arrangements from the sky. Then, coins that seem to be leaping up from below, much like a jumping Cheep Cheep, can be found. More coins then fall from above along with a giant coin. This is followed by a Red Ring with several coin rings that move back and forth. Collecting the eight Red Coins will earn Mario a Gold Flower in his inventory. More coins then begin to fall down to a large abyss, where Mario drops down and ends the level.