I'd just like to point out that it's not terribly wise to assume things such as this, be it the PS4 being a failure or a success.
Take this console generation for example, given Nintendo's track record, nobody expected them to dominate sales as much as they did. Of course, you could easily make an argument that just because the Wii sold such a ridiculously large amount and was a success, it still wasn't a console that was worth owning due to lack of worthwhile games.
The Xbox 360 got a year head start over the PS3 and many assumed that the PS3 would die and push Sony out of the console business. What's happened now however, near the end of the generation? Sales of the PS3 worldwide have now overtaken the Xbox 360, pushing Microsoft back into 3rd place yet again.
Of course, this could change once more and maybe Sony will end this generation in last place, but placing bets on who will succeed and who will fail will rarely provide the expected results, and this console generation is absolutely proof of that.
If you prefer the Forza Motorsport series however and are frustrated by Sony and PD's handling of the Gran Turismo franchise, then of course it makes sense for you to invest in the next Xbox, especially since it's pretty much confirmed that the new Xbox will launch with Forza 5 near the end of the year. Since Microsoft has officially stated that the Forza franchise will see yearly releases (alternating between Forza Motorsport games and Forza Horizon games) then you'll likely be very pleased with the regularity of releases
All I will say however not to make any assumptions as to which consoles will succeed and which will fail, since there are far too many variables at play.
Personally I'm very curious to see if the PS4 will actually have a smaller GT game within the first year of launch. I can't help but wonder if Sony has looked back at the tremendous success that the PS2 saw when it rolled out the gate with GT3, and instructed PD to focus on developing a smaller, less content heavy GT game to help shift units at launch. I imagine Sony would see very impressive sales if they pushed the PS4 out accompanied by new Killzone and Gran Turismo games!