Premium Tracks are only available in Bloons Tower Defense 4 and Bloons TD 4 Expansion. They are available for purchase with NK coins through through a tab at the top of the screen when you click "New Game" at the Main Menu. There are 4 difficulties of track: Beginner, Intermediate, Advanced, and Expert. There is also a Track Pack available for purchase.
Here is a list of all the Premium Tracks:
Description: An easy and fun BTD4 track where the bloons must pass through the central area at least 5 times before heading to the exit.
Description: A simple track where bloons can come from either direction. The large area in the centre is a good spot to build most of your towers.
Description: This is a moderately challenging double path track. The Bloons on the left most track come from the top of the screen, and for the right track, from the bottom of the screen.
Description: A simple track with a twist - the bloons criss cross the central path multiple times making it easy to get good value for your towers. However there is limited build area in the upper part of the track and the bloons can come from either entrance!
Description: This track is bizarre and highly challenging. It has 4 separate tracks that start on the top half of the screen then loop once around the centre, finally heading to their respective exits.
Description: This is an extremely challenging track. Two separate track converge then split then converge then split before heading to the exits. Although there is lots of good building area, the tracks are shorts giving you no room for error.
Description: This track is one of the most challenging you can ever play. It has 5 very short separate tracks that combine into one. Good luck, you will need it!
The Track Pack gives you all 7 purchasable tracks in one go. Buying them in this manner saves you 30!