You can try it in the second memory slot and see if it works. We see a lot of memory problems mixing memory, so its 50/50 on whether it will work.   Best to have two of the same brand and model.  If both do not work, install the new 16GB in the 4GB slot and consider adding another same 16GB Cruical memory. Having two memory modules allows dual channel operation (which probably helps speed).   There are many options for this model, entry level Intel Celeron CPU up to a 9th Gen i7.  CPU can be a source of slow operation.  Another potential is the system drive that can vary between (slow) mechanical hard drives to a (fast) M.2 2280 NVME SSD.  If you have mechanical hard drive, consider upgrading to SSD as it will speed up everything from startup to operations.
I bought my Inspiron 3470 from PC World just before covid arrived and it's always been slow switching between Firefox / Excel / iTunes / Thunderbird. Task Manager / Performance shows I have 4.0GB RAM, and in Command Prompt, "wmic MemoryChip get" shows Speed is 2666 (although ConfiguredClockSpeed is 2400). I've just bought a single 16GB RAM (Crucial CT16G4DFRA266 DDR4, 2666 MT/s, PC4-21300, DIMM, 288-Pin). Should I add it alongside the existing 4GB, replace the existing 4GB, or put the 16GB where the 4GB is and the 4GB in the other slot?
Can I add 16GB RAM to 4GB?