Engagement program streams will always send the topmost unsent email first. So if the stream has already sent out Email 3 and Email 1 is only available tomorrow, then Email 2 will be sent next. Tomorrow, Email 1 will be sent since it is only available tomorrow, followed by Email 4 the following day.
Make a note that engagement streams will only send out the same email once if it is sent from the stream. If a person is sent Email 3 from a smart campaign or email send, they will again receive Email 3 from a stream, but only once in the stream.