If I add memory will it help High Sierra function better?

Absolutely nobody is going to be happy trying to run a modern macOS (including your High Sierra) on 4GB of RAM.

https://eshop.macsales.com/shop/apple/memory/

http://www.crucial.com/usa/en

MacBook Pro (15-inch Mid 2012), macOS (10.13.1), i7 480GB SSD 16GB RAM iPhoneSE iOS10.2, Parallels13.0.1, HP OfficeJet Pro 8620

It's very easy to determine if a Mac's performance is being constrained by available memory. Please refer to Use Activity Monitor on your Mac - Apple Support. If the "Memory Pressure" graph is frequently "red" your Mac would benefit from additional memory. If not, it won't. It's that simple.

... my memory cleaner

Rule 1 of Macs is don't install junk. Uninstall your memory cleaner.

I suggest you use the program created by Etresoft, a frequent contributor. It will provide a snapshot of your system which we can analyze to possibly determine the cause of your problem. Please use copy and paste as screen shots can be hard to read. On the screen with Options, please open Options and check the bottom 2 boxes before running. Click “Share Report” button in the toolbar, select “Copy to Clipboard” and then paste into a reply. This will show what is running on your computer. No personal information is shown.

Etrecheck – System Information

Hello astrophee,

The first thing to do would be to get rid of your "memory cleaner". Modern versions of macOS, including the latest version, High Sierra, are designed to efficiently use all of your RAM. Using some 3rd party tool to interfere with that design is only going to slow down your system.

You didn't say exactly what kind of machine you have, so it is difficult to give accurate advice. However, I can tell you with certainty that if you could upgrade the RAM on your MacBook Pro, then you can also probably replace the hard drive with an SSD. You will get a far greater performance improvement with an SSD than you would with a RAM upgrade. I am not talking about a percentage increase either. An SSD should make your machine run about 4 times as fast.

If you do open up the machine to install an SSD, then you may as well upgrade the RAM at the same time if you can afford it. But because I don't know exactly what kind of MacBook Pro you have, I can't tell you how much RAM you could install. Some machines are limited to only 8 GB RAM. I'm not entirely sure if machines of that vintage could or could not run High Sierra. There is no MacBook Pro that can be upgraded to 32 GB RAM.

Also, there is a comment above that claims a recommended download from a non-Apple site of a program that starts with E is infected with malware. That comment appears to refer to my software EtreCheck. I don't why that comment was made, but I can guarantee that EtreCheck is not packaged with malware. EtreCheck is packaged with caution about advice posted on Internet forums like this one. When in doubt, always ask for a second opinion.