Windows dynamically manages graphics memory, so games can automatically use up to 4 GB RAM. Some games check amount of dedicated (onboard) VRAM, it is possible to change that amount reported by driver, but this unlikely will result in any performance improvement (more likely game settings will be at unplayable level) .
It is possible to increase max turbo frequency of IGP by BIOS editing, but actual frequency depends on CPU temperature, so unlikely it will reach desired value - CPU will overheat and drop frequency earlier.
Frequency by itself is not a metric of GPU/IGP performance - the shader cores amount have much more value. So even having twice MHz of 920m but only 1/20 number of shader cores (at best) means that IGP is still a crap in comparison with dedicated card (in games and CPGPU, video acceleration is different thing due special blocks in GPU/IGPs)