If you want DDR4 right now, you have to go X99, which means the 5820k.
Really though, an overclocked 2500k will perform as well as a 4790k in most games. Are there any games in particular you're not happy with the performance of?
There are currently no mobo's that support both DDR3 and DDR4, so you're right. To use DDR4 you need to use X99/Haswell-E. Maybe with Intel's next consumer socket they'll have mobo's that use both DDR3 and DDR4 but that's just speculation.
Damn, I didn't know DDR3 was not already backwards compatible.
My main reason is it seems like in any type of open world game or any CPU intensive games, my system seems to struggle. I have an R9 290 and when I compare my performance to benchmarks with better CPUs, they seem to have significant gains over my system. I have my 2500k overclocked to 4.2Ghz on air cooling and I don't feel as if pushing it another .2 or .3 more Ghz will make much of a difference.
Seems like I should just wait and see how Broadwell and Sky Lake turn out