5, by Mario Gotze against Manchester City!! won that game 7-2
Apart from that, Luis Suarez scored 4 goals against Manchester United, that ended 5-1
Carlos Fierro also scored 4 but against Southampton, that ended 6-0
These have all been in my 2014-2015 season, before that I think Dirk Kuyt got 4 goals against Manchester United back in the
2012-2013 season, I think that was like 4-3 or 4-4, can't remember was a while back.
Edit : Taking regen's into account, my 15 year old Portuguese striker Hugo Silva scored 7 against Stockport U18's, so overall he'd be the player who's scored the most in one match for me.
He looks promising, has scored 36 goals in 25 games for the U18's and reserves with a 4-star rating.
I've only seen one 4.5-star rated regen in the game which I bought, an English Central defender called Calum Konway, West Ham's youth system made him, he's brilliant and 18 now, in my first team already, cost me 10,000,000 at 16 years old!
Money well spent though.