4 weeks after the transplant (3500 grafts), hair doesn't fall out but doesn't grown either at the front.

Transplanted hairs fall out in a few weeks.  New hairs will grow out in 6 to 12 months.  You may see some growth at 3 to 4 months.

Many patients are concerned about whether their transplanted hairs fall out or not.  For the great majority of patients, all of the transplanted hairs fall out within the first month.  (The hairs actually break off at or near the surface and the follicle remains dormant for a month or two under the skin)  Now, this is normally what happens.
There are many variations on this theme.  Some patients' transplanted hair continues to grow for a while and then stops for a while again prior to growing normally.  Other patients have all the hair fall out at once and regrow in 3-4 months.  Some lose some hair and the other hair remains but doesn't grow for a month.  All of this is also normal and it has no bearing on whether the surgery was a success or not. 
I would recommend keeping the scalp clean and making sure to avoid excessive sun exposure this summer.  Follow up with your physician in 6 months to see how things are going.  At this point, everything sounds appropriate.

These hairs often stay for a while but they will eventually shed.   They are never a problem for new hairs that start to grow. Usually, there is a lag time between the time the hairs shed (2-8 weeks) and the time the new hair grows (3-8 months). IF for any reason, there is an overlap between the two, pay no attention to it for when the new hair grows, the old hairs from the grafts that may persist beyond 8 weeks, will eventually shed