The level codes are:

04-“HAHA” 12-“REAP” 20-“GROW” 28-“YALI” 08-“GRIN” 16-“SEED” 24-“TALL”

For a 4000 point bonus, you need to get the flooze to cross itself five times. But remember not to get too cocky, and wait until the end of the pipe to do this. There is also a secret bonus of 50,000 points if you completely fill the screen with pipes.