DL4 is a work around. It's really a good one. I got my endorsements by asking for this exemption too.
Anyway, here's why you need to fill DL4 form:
You are considered to be a professional driver after you have hold your full NZ drivers license for two years. I believe you have had full drivers license for more than two years from another country. When you fill DL4 form, you are asking NZTA to exempt you from waiting two years by proofing that you have had a previous driving experience in overseas, and convince NZTA you never breach any NZTA rules, such as never overspeeding, never caused or involved in an accident, etc.
Let me know if you have further questions.
Cheers!
Sackboy