Four quarter notes is not a very long percussion accompaniment, so let's make it more interesting.

Open the first percussion segment in the standard notation editor by double-clicking it.

From the menu select 'Edit > Select Whole Staff'.

Then from the menu select 'Adjust > Notes > Eighth Note'.

This compressed the notes, which isn't what you wanted. Undo the change with 'Edit > Undo'.

Again make sure that the whole staff is selected.

From the menu select 'Adjust > Notes > Eighth Note (Without Duration Change)'.

Now there are eighth-note rests between the original rhythm. You can add off-beat accents as you wish, in the space where the rests are. You can switch to the percussion matrix editor, or stick with the notation editor - it can be fun guessing which pitch represents which instrument.

Now that you've made a change, you'll have to delete all the copies and re-copy them. We'll use a better solution instead, but you'll still have to start by deleting all of the copies of the segment. Remember to keep the first one!

After removing all but the first segment, from the "Segment Parameters" box in the left toolbox, check the "Repeat" checkbox.

Now the segment repeats forever. This can be useful, but eventually you'll want to stop the song. When you want to change the repeats into copies, from the menu select 'Segment > Turn Repeats Into Copies'.

Adjust the first track so that it's repeated, too.