We have to think over the problem..
  Actually..~the problem is so easy but.. some of us got WA at test#4(like me..)just because we hadn't thought about whether we could walk through the labyrinth from one exit to the other..

We can enter the labyrith from the lower right entrance as well as from the upper left entrance. It seems to be the solution for our problems in test #4.

aha,,, Good insight~ : )
finally, I got the reason why Test#4 is valid.
Thankx a lot~