4GB RAM is pretty small for any current OS.
1TB is storage, not really "memory".
Disk utilisation of 100% is probably down to fragmentation and paging. Paging out can be reduced by adding RAM, fragmentation by defragmenting the HDD. Note that you shouldn't do this with SSDs. Before defragmenting, delete anything you don't want, un-install programs you don't use and user accounts you don't need, then empty the recycle bin.
If you are concerned about performance, the 2 best things to do would be to double the RAM (or more) and switch to an SSD for your boot drive. You don't have to ditch your current drive - use it for storage.
It would be easiest to clone the current HDD to the new disk, that way you wouldn't have to reinstall your OS and applications and transfer user settings. The down side is that you would have to have a new disk at least the same size as the used space on your existing disk, so you might have to get a 1TB disk: £50 - £90. Probably have to also get a 2.5" to 3.5" caddy to install the disk.
That's a pretty old CPU & chipset - launched in 2012, EoL 2019, so DDR3 RAM - cheap as chips! £20 for 16GB (2 x 8GB modules) on Amazon, possibly cheaper 2nd hand / refurbed on ebay.
For £70 - 100 you could vastly improve performance.