Yup... To keep dreaming
5 GHz on air isn't very realistic.
As for hitting 4 GHz then as you're so close, there is probably some tweaking you could do to get you there. You said your temps are ok, so you could put a tiny little bit more voltage through it to get you to the 4 GHz target but you wouldn't notice any performance difference compared to what you're at now. With higher voltage comes higher temps too so if they start to go up then you might want to look at a higher end heatsink and fan. That said, there is always the chance your CPU is at it's wall then that's that really.
CPU: Intel Core i5 2500K @ 4700mhz - 47 x 100MHz Mobo: MSI P67A GD65 GPU: EVGA GTX 570 RAM: 2 x 4GB Mushkin LV 1.35v 1600MHz DDR3 HDD: 3 x 1TB Samsung Spinpoint F3's Case: CoolerMaster ATCS 840 - Custom House of Kolor paint job. PSU: Corsair HX750 Cooling: Custom WC - XSPC Delta V3, PA120.3, DDC Ultra with XSPC Res Top, 3/8" XSPC UV Green Tubing over 1/2" Barbs OS: Windows 7 Enterprise Edition 64-Bit