4, 5 and 6 looks like a numbers too, so.. why it doesnt work ?
Look at the code that works. You compare currentMillis1 to previosMillis1, and then update previousMillis1.
Look at the code that does not work. You compare currentMillis5 to previosMillis5, and then update previousMillis4.
You need to explain why you think that should work.
If you need to perform one of 6 tasks depending on what time it is now, how many times do you need to look at your watch? ONCE! Not 6 times.
Should I change millis() to millis1() 2, 3 etc ?
You can certainly try, but there is only one millis() function, like you only wear one watch, so you can probably guess how well that will work. If not, try it.