United paid Atletico Madrid ??18.9m to take De Gea to Old Trafford but he initially struggled to cope with the physical demands of the Premier League.
The then 20-year-old was placed on an intensive strengthening programme which paid dividends and the Spain international is now widely recognised as one of the top keepers in world football.
“I have been here for four years now and I think that every aspect of my goalkeeping has improved,” De Gea told United???s official website.
“It is hard to single out one area because we have worked and improved on everything.
“I think the fact that I have started to do interviews in English has really helped as well, it has helped me to learn more of the language as well.”
Outstanding displays from De Gea helped United defeat rivals Liverpool 3-0 on December 14, while another influential performance secured a point from their goalless draw with Tottenham on Sunday.
But De Gea insists he can improve further as Louis van Gaal???s side try to retain their place in the Champions League places.
“Again, no matter how excellent your form is, you can always get better and there are always aspects of your game that you can improve,” he added.
“You can improve everything. No matter how well people think you are playing, or how well you think you are playing yourself, it is always a case of working on every facet of your game.
???There is hopefully more to come.”