Best upgrade for Crosshair IV Formula is X99-A USB 3.1/5820K and 16GB GSkill DDR4 for 105$.
Better than that not exist.
OK C4F is capable to hold newest AMD CPUs but not worth investing in that.
It's success if you sell everything and cover one part of X99 budget board and investment is CPU and memory.
X99 budget boards are 190-240$. GSkill Quad kit 2800 is arround 105$.
You see what is better almost 300$ for i5 or arround 380$ for i7 6 cores.