@ysmina-- Are your tablets scored? That is, do they have a line in the middle? You can use that line as a guide to cut them evenly into two. Yes, cutting them into quarters are tricky.
If you need to reduce the dose less than a half or a quarter, you can do that by dissolving a whole citalopram tablet in a bottle of water and then measuring the water/dose. For example, if you want to take a quarter, simply pour out half the bottle, and then another half. You can pour out another half to get half a quarter dose which would be 1.25mg.
This is a good way to reduce doses slowly for people who have a lot of citalopram side effects.