Have donor area less dense. Should I go for FUE as 4000 grafts needed? Follow-up. (Photo)

Your donor does look a little bit thin however the ultimate results available donor hairs is indeterminate. But the principle is to take as many from scalp first before going to the chest.

You are bald or balding to a Norwood 6 ot 7 based on photos.Before embarking on such a large number of grafts, you need to have a realistic expectation of what 4000 grafts will produce.  It will never give you the hair you once had.  You need a master plan.

As you think about chest and scalp hair FUE, keep in mind that only about 50 % of chest hairs survive. (the survival is much lower than scalp FUE where the number is well above 80%). In order to cover your scalp with nice density, you'll need about 9000 follicular units. Given that you don't have this many, you'll achieve a thinner coverage with anything less than this number.  Many patients choose to focus on the front half and build nice density in that area and either not transplant or sparsely transplant the back (crown). Without seeing your scalp up close, I can't comment on an appropriate amount to move. Certainly your donor is thin but it really depends on your current age and how much thinner it could get. It's critically important to be aware that building nice results with limited donor hair is a real art. 500 surviving chest hairs (out of 1000) doesn't add a whole lot. 

Class 6-7: If you
have high quality hair (average or better weight) some salt and pepper color
and enough donor density, numbers in the 5000-7000 grafts will be needed. Even
with these numbers, the fullness throughout the scalp will not be 'full
looking'. We are now combining less grafts (like 4000-5000 grafts) with Scalp
micropigmentation which will give you a full look. See the website below. In
addition, go to my website and see over 1500 patients, many with patterns
similar to you listing the number of grafts to achieve the results you see
there at newhair website under galleries.

If you have selected an experienced hair specialist, the doctor can tell you all the pros and cons for each harvesting technique. They should also be able to recommend the technique based on your individual case. Since each case is different, all the possibilities have to be discussed and examined before making a decision.  In your case since it is a large recipient area with limited donor hair to get the maximum result from the donor hair available, I recommend doing FUT technique. It is more efficient and less collateral damage.