As others have mentioned, more information would get you more accurate answers. Your original post indicated he worked at a daily rate and, for $4000, he expected it to take 2 days. That is $2000 a day, which, even at 10 hours a day, would be $200 per hour... seems high (or something is off in the original post). Personally, I have never generated a bill of more than $1200 for milling by the board foot in a long day.
If he is working for a daily rate, and has that much time in (and you have settled up with him), you've spent most of your budget for buying a mill. Yes, you can purchase a new sawmill for $4000, but what you get won't likely be anywhere near as capable as what he may have been using, and what other sawyers would be bringing to your site.
If you intend to continue using your mill for projects, long after this particular batch of logs (trees) is done, it may be a good investment. If you are anticipating milling for hire after your project is done, your budget is woefully inadequate. If you intent is to get this batch done and you'll have enough wood to last a lifetime, hire someone else to get it done.
I don't recall seeing your location, which may be a factor in finding someone else. If you decide to buy a mill, consider looking for a used mill. They hold their value fairly well (if maintained) and if you can find one with hydraulics in your budget, it will be a wise upgrade. I found mine, 18 months old and with 5 hours on it, for just over 20% off new price, through Sawmill Exchange.