Harry was born in 1980. Therefore, the fourth book should be set in 1994. Yet, Harry mentions in his letter to Sirius that Dudley chucked his new PlayStation out of the window. The only problem is that the PlayStation wasn't on the market in Britain at that time.
"As obsessive fans will tell you, I do slip up! Several classrooms move floors mysteriously between books and these are the least serious continuity errors! Most of the fansites will point you in the direction of my mistakes. But the essentials remain consistent from book to book because the story has been plotted for a long time and it is clear in my mind."
—J. K. Rowling on the errors within the franchise
Dudley smashes his PlayStation before it was even released in Japan (Dec '94). This is simply an error on JKR's part, as I doubt she knew the history of PlayStation when she wrote GoF.
The other possibility is that Vernon had signed up for the product tester, and received a PlayStation and a few games before the official release in Europe (Sept '95).
As the other answer mentions, Vernon might've signed up as a tester. But a tester of what ? It could've been one of the Nintendo PlayStations and then it'd be perfectly understandable to chuck it out in frustration because it was a prototype and those are usually not the most reliable. These were made in 1991 so the possibility is there -- although of course in 1991 Nintendo already broke up with Sony so the chances of one of these finding its way to England in 1994 to a non-industry person are fairly slim but maybe magic could explain it...