HAHAHAHAHA!!! HAL played a six... (and remember that Adam didn't have a single six from the beginning? :-)) So, Adam will have to hit his own 4!
Notice that regardless of the fact that Adam passed on 6, at this point you could know tha neither Adam nor anyone else has any other six: you have the last six! (just count them: there are six 6's on the table, and you have one 6 in your hands: that makes a total of 7, so there aren't any more 6's in anyone else's hands).
In this case, your six is what is called a "head". That tile is an absolute guarantee that you won't pass as long as you have it in your hands (because no one else will be able to block that 6 -- no one else can have a 6 to play!). Thus, you should not play it if you have the possibility of playing something else (that would be blocking your chances to play later!)