try the first wooden bridge you see going from the west save point and going eastward.
break it with ball 3.
play the level that appears.
that gets you ball 5: :a5:
Utopica Minor is totally optional, btw
also, btw, in utopioca (& minor), things may not be the way they seem, but not exactly invisible. :P
it's more like things that you can see on screen are mirror images of what the actual platforms are (in minor, if I recall correctly).