I remember reading somewhere (I don't know where) that a rest counts as a musical accent. Is this true?
If so, then for example, when can I rest in 4/4 for an instrumental piece?
Second half of your question first. "...when can I rest on a 4/4 instrumental...?"
Anywhere you like. As witness any piece of notated music, where the solo part WILL contain rests.
Now we try to discover what "...a rest counts as a musical accent" means. Maybe just that you keep counting, whether there are notes or rests.
Or maybe it's a bit more philosophical. If there's a strong beat underlying the music, it can said to be be felt even when a note isn't actually played ON that beat. "The silence was deafening."
When singers tend to run into the rests, I sometimes say 'Sing the rests louder!'
An accent on a note makes that note played with more emphasis than the surrounding ones. So, from an actual situation, it would be impossible to make a rest sound more emphasised (louder) by writing it with an accent (>).
However, in the majority of music, we feel beat one as the usually more emphasised part of most bars - without that, it's pretty difficult to say what the time signature would be. So - if that particular part of a bar was missing sound - it was a rest - the listener would likely expect something there, thus making it a small period of time within that bar that was 'emphasised'. Rests are after all part of music, which will rarely be sound after sound after sound. Deafening silence?
That said, there is no deafinite place where rests (of any duration) are supposed to be; they fit where they go, due to the whim or choice of the composer. But, they will always be part of the music (or lack of it), adding to the final effect.
Rests count as silent units of time and rhythm in music depending on their duration just like their note counterparts (i.e. half note/half note rest) but they are not accents. They also don’t have accents placed on them like notes can. After all, how can someone accent silence?
As far as where to rest on a 4/4 instrumental, the placement of rests are choices just like the placement of notes. There is no formula to say where they go, they can be placed anywhere one chooses. They are used in a way like commas, periods and paragraphs are used in language. The person who writes or improvises the music decides where they go.