أعط أجوبة - English

When you say eg.: I haven't eaten since 4am, means you ate at 4 last time, and no more since then.

When you say : I ate 5 hours ago, you refer to one point in time. And depending on the stress this sentence can have 2 meanings.

One is that the last time you ate was 5 hrs ago, and that you were eating at that exact moment.

And with since you use an exact time, like May 4th for example, and with ago you say a length of time, like 5 hours, or 2 days sth like that. I hope you understand xD