Hi
@McJames,
List out the specs for your system please. 4 years isn't that bad. My daughter games on my Secondary AMD system below (sig) which is 11yrs old and we play BF1 & BL3 together almost everyday so... no worries there as far as age. (That's not to say a parts isn't going bad as a new part can be DOA)
So your PC at times won't boot to your desktop. How often does this happen and can you "FORCE" it to happen? Does anything come on the screen when you 1st press the power button? When was the last time you reinstalled windows? Have you cleared your Cmos via the jumper or by removing the battery for 10mins? (Sets Bios settings to default)
Did it just stop running one day or did you install a new part or it was dropped/tipped over or a friend borrowed it & now it doesn't work...? Does it have a SSD or an older HDD?
If it's been sitting for 4 years, was it left in the garage or moved around? If moved around, possibly a cable came loose. Have you completely tore the PC down to base parts and then put it all back together part by part? Making sure everything is connected correctly? Have you swapped parts from 1 PC to another to test if they work? (PSU/RAM/GPU etc.)
A little more info please.