deleting rests

Generally, you can't delete rests in voice 1. You CAN delete rests in voices 2, 3 and 4, but it's actually better just to them invisible. (toggle visibility with "v") That way if you are editing those voices, you will have fewer problems with rhythms and rests in those voices.

If you explain better what you are trying to do, we'll be able to give you a better answer. Deleting a rest and making the measure shorter than the time signature is the exception.

Indeed, the answer depends entirely on why those rests are there and what you want instead, so we'd need to see you score in order to understand. If there are too many beats in the measure and you want fewer beats, select the rests and press Ctrl+Delete. But if they are there because you have content in other voices and you don't need to see the rests in this voice, chances are you really should be using voices differently. For example, a common mistake is to enter the tenor and bass parts in voices 3 & 4 on the bottom staff in an SATB arrange, but that's not correct - you should always start with voices 1 & 2 for each staff. Then there are no rests to delete. In which case, what you really need to do is move the voice 3 notes to voice 1.