How much thrust is required for a Boeing 747 to "keep flying" at 4 km altitude?
The QRH (Quick Reference Handbook) contains a few tables for flight with unreliable airspeed. The one for holding comes closest to the situation you asked about:

(Boeing 747-400 QRH 20.2 - Performance Inflight - CF6 Engines)
As you can see, holding at 10000 ft altitude requires between 59.7% and 77.4% N1 depending on weight. You asked about 4 km, which is about 13000 ft, so a little more will be required.
There is also a table for cruise speed at various altitudes:

(Boeing 747-400 QRH 20.1 - Performance Inflight - CF6 Engines)
You can extrapolate the values shown here between 10000 and 20000 ft altitude to get the required thrust at 4 km. Note however, that the speed flown here is higher (290 KIAS or Mach 0.84) than the one required for maintaining altitude.
If you combine the information from both tables, you should get a good idea of the required N1 in your situation. The number will however strongly depend on the weight of the aircraft.