16+4 means that only 8G out of the total 20 will be running at full speed.  You're better off with one 16 than you are with 16+4 (the single soDIMM will run entirely in dual channel mode rather than just 8G with the rest at half-bandwidth). The CPU controls the memory speed -- RAM faster than what you now have will simply downclock to the current speed.
Hi, I want to upgrade my laptops ram I have 2x4 gb ram  and I want to make it 16+4 is it possible? Dell's guide about g3 15 3500 says max ram is 16gb but command prompt and Crucial's web site ([URL] says max ram is 64. So can I do 16+4? And my rams are 2933 MHz but 16gb ram ill buy is 3200mhz, both will work in 2933 MHz and there wont be any problem right?
Dell G3 15 3500 RAM Upgrade