Oktas8,
In the Boeing 707 airplane I now fly, flap retraction is initiated at a certain altitude (500 feet AGL or obstacle clearance altitude, whichever is higher) and airspeed (V2+30 knots on initial takeoff or Vref+40 knots for a go-around). It's the same procedure for four or three engines operating. Hope this helps. Take care and fly safe.
SentryIP