I would say 8 GB minimum. New PC's will come with Windows 10, 64 bit OS and the 64 bit OS uses more of the basic memory than a 32 bit, thus less is left for applications. Even 8GB may be questionable depending on how much synth files are used.
I recently bought a new PC (for on-site recordings) and looking at the ones with only 4GB, they were not really what I would call a "DAW" type laptop, more for consumer use. The one I bought was a Dell Inspiron 15, 5577 which is sold as a "gaming" laptop. It has a 7th gen i5, 8GB of RAM (upgradeable) and came (standard) with a 256GB M.2 SSD. I removed what I didn't want and it runs clean with the finicky Resplendence Latency Mon. It boots up and loads programs (visually) as fast as my desktop DAW that has an i7 6700K CPU.