We mean to say ending quality 4 or 9 and thats the 2nd or third number of player's quality.
For example if a player is 3
4Q or 3
9Q thats a player with endining Q 4/9.
ending Q
4: 1
4 2
4 3
4 ....13
4 and so on.
Ending Q
9 1
9 2
9 3
9 4
9...11
9..and so on
So basiclly a player that's 5Stars and has ending Q of 4 or 9 is really worth to buy as he'll get next star pretty fast.
An example of player with 34 Q that's really worth bidding for.

Young
high price compared to other players that are 19y.o with 34Q, he's around 4mil while other guys are 2-2.5mil, higer price players train alot faster than lower price players.
Note: Training fast does NOT always mean performing good. Some fast trainers would often get 5-6 Ratings for a match, others would get 8-9 so price isn't performance factor.