It appears from the DL skeds that another (5th) 744 is nearing modification completion. The sked is showing an old configuration from NRT-SIN on 21May and a new one on the reverse route on 22May.
Aircraft Reg# should be N666US which left service around April 7. N661US went out a couple of days later, so that one may be returning very soon. N665US went out of service arouind 23Apr and if following the accelerated skd, should return to service during the 1st week of June. At that point 7 of 16 aircraft should be completed. This is assuming that the aircraft out of service are in SIN for cabin mods only.