A while back I got into D&D at a young age but didn't get far into it before it faded from my mind. Recently (last week) I got back into it after watching some YouTube videos which cleared up a lot of issues I had with it at the time. That being said I bought a D&D book a few months back for forum roleplays I was into at the time. The book is the Player's Handbook for the 4th Edition. Since the latest is 5th Edition and it will be a bit of time before I can get a copy of the three books (Player's Handbook, Dungeon Masters Guide and Monster Manual) would it be efficient to read the 4th Edition Player's Handbook to learn how to play D&D or would that be a waste of time considering how much I've heard about the difference between 4th and 5th edition?
No. 4th Edition and 5th Edition are completely different versions of D&D, and share very little in terms of rules and mechanics. If you're looking to learn 5th Edition, I suggest you start with the freely available D&D Basic Rules, which include almost all of the game's rules, but only a small subset of character creation options.
Not really. While 4e can be a good starting point for people who are new to tabletop rpg's, the 2 editions are too different to really transition between them fluidly. There are very few transferable skills between them.