Right now, $4000 for a month should be oodles, but prices change both up and down so fast. 1,300 for the trek, 1700-1800 air fair, 900-1000 for tips, extra meals, souveniers, etc. I have seen the airfare go from 900-3600 and back during the 4-5 years I have been watching. But the 900 was during the political era.
The trek fee is from pick up at airport to drop at airport, 15 nights in the tea houses, 3 nights in hotels, meals and porters to carry the stuff and the shuttles to and from the hike for 12??. Can't remember the exact numbers. We have to buy our own meals on the layover days in Kat and Pokhara. Also, the air fare could be much cheaper giving the 3 +/- days option which would increase days in Katmandu at own expense. But as of last week, $4000 would be plenty. There are also many, many tour operators with different rates. Also, if we went as a group, we would have to decide on options, such as side trips into Tibet, river rafting, etc.