When quality ends in 4 or 9 they are 1 level below the next star (maximum of 15 skill points below next star). Lots of people like to go for these in auctions and so if you sell your players when their quality ends in 4 or 9 then it is more likely that bidders will keep bidding and you will get a higher price for them. However it's not certain by any means, likewise it's just as possible to have this happen with a player whose quality doesn't end in 4 or 9, albeit less likely.