Well, 4 years won't really be considered as "new" but that might depend upon when your last examination was. I do personally think 4 years is time enough to have a plan well in play. When a change to the next size bank occurs from organic growth, exam teams usually expect the bank to have been preparing for that change and will examine the bank as an ISB.
Re your exam, you can contact the exam team and ask how they intend to approach the examination given the recent change in category for the bank. They should already know how far back they plan to go and how they will approach the ISB requirements with prior years.
Changes are part of your performance context. Document when the assessment area was formed, what was done to address the needs in the new AA, what was done to meet ISB requirements, and what is planned going forward.
Look at other banks in the AA of similar size and product set, reviewing their examinations to see what is considered satisfactory in each category. Note any differences with your bank and set a goal.