Parts for desktopMaybe if you offered a LITTLE BIT of information, like what your hardware specs are, motherboard, CPU model, etc., and what you are doing with this system, somebody might actually be able to give you an intelligent answer. Otherwise, you're going to get a pretty generic guess from left field.
should i go for 4000mhz?You 100% want to stick to a 2 DIMM kit unless you absolutely have to have more memory than you can get with two DIMMs, for that dual channel board.
If you need more than 32GB, or suspect you might EVER need more, then fine, you can try a four DIMM kit but you may end up having to drop the memory speed a bit manually or increasing the SOC voltage to get it work. If you don't need more than 32GB, and if this is primarily a gaming machine that won't be running very high end, complex, professional applications or VMs, then you don't, I'd highly recommen sticking to two 16GB DIMMs AND make sure they show as compatible with your motherboard using the G.Skill memory configurator.
Anything listed on these two pages of results should be compatible.