We use those modifiers a lot and for most learners they are confusing at the beginning. They generally say sth about the speaker's attitude towards the content of the sentence: e.g. Es ist 4 Uhr. = pure information. Es ist erst 4 Uhr. = From my point of view that is rather early. I thought it was later. So it may imply there is still a lot of time. Es ist schon 4 Uhr. = It,s pretty late. It might imply sth like Finish your work; or get ready if you don't want to be late - depending on the context, of course.