Event edit questions

The byte after Begin Action queue tells the game how many bytes does it have to read for the actions of the selected actor. This is because some bytes can do different things weither they're in an action queue or not.

90 outside an action queue unlocks the Bum Rush, but inside an action queue it moves vehicle/entity up 5 tiles.

So 04 tells the game there are four bytes (three + FF, end of queue) the game has to consider as action queue. There are variations.
04 tells the game to continue with the following code without waiting for it to finish. 84 (80 + number of bytes action queue is long) would tell the game to wait till the action queue is finished before continuing with the rest of the code. 00 + "number of queue bytes long" is useful if you need to have a two or more characters moving at the same time.

For the second question, I don't think there's too much relation between adding 01 to the resulting action. When I event edit I just search in the dump "up 5" or "right/down 3". It does the trick better than trying to memorize. I still don't know what byte to put to make a character face left. I always check it out.