FUE in 2 Stages?

4000 grafts may make your donor area look very thin especially if you keep your hair short.  There is no way to work around the fact 4000 FUE as it is taking 4000 grafts from the donor area.

Most
people with healthy donor area can easily support 4000 grafts or more for their
transplant.  It does not really matter if
the two days are back to back or with a gap. 
Many people in our practice do it back to back just to get done and over
with.   That way the swelling, redness, hair washes and
other post op problems are all at once.  You
need to talk to your doctor to see what he recommends and whether or not your
donor area can support the numbers of grafts.

You do have a valid concern. First, I should mention that you should not do all 4,000 grafts in one day it should be definitely split. In my experienced, out of 1,400 cases that I've done, I generally I do not have a significance problem. In fact, I have perform 20 patients over 5,000 grafts each and I will publishing a paper that will show not a significant thinning in donor area. I use a product called Acell. This a growth factor from the inner lining of pig bladder. Although not magic it promotes better wound healing. I used  this in almost all of my FUE cases I have had very good results even though there is some thinning in the back of the scalp I have never had any patient who complained about it yet so far.