Should I change to my new tray that has attachment slots even though I don't have the attachments on yet?

The best option would be to make your appointment sooner to have the new attachments placed otherwise it will slow down the progress of your treatment.

 Yes you can wear the trays that are made to fit over the attachments without actually having the attachments bonded on yet. As long as the 3rd tray was fitting well and the 4th tray is completely seated with no spaces at the edge of the teeth, you can move to your next tray.  Please note you may have to wear that tray again for 2 weeks once the attachments are placed or call the office and move your appointment up two weeks

Yes, you can go to tray #4 with the attachment slots even though you do not have the attachments on your teeth yet. Do not go onto tray #5, however, until the dentist sees how #4 fits after you receive the attachments.

The answer is yes, you can wear the trays that are made to fit over the attachments without actually having the attachments bonded on yet.  So as long as your 3rd tray was fitting well, you can move to your next tray although you may have to wear that tray again for 2 weeks once the attachments are placed.