AP_NavEKF: change using_gps threshold to 4s

this means plane will continue to use dead reckoning with GPS 3D lock
for 4s

libraries/AP_NavEKF/AP_NavEKF.cpp

@@ -4803,7 +4803,7 @@ void  NavEKF::getFilterStatus(nav_filter_status &status) const
     status.flags.takeoff_detected = takeOffDetected; // takeoff for optical flow navigation has been detected
     status.flags.takeoff = expectGndEffectTakeoff; // The EKF has been told to expect takeoff and is in a ground effect mitigation mode
     status.flags.touchdown = expectGndEffectTouchdown; // The EKF has been told to detect touchdown and is in a ground effect mitigation mode
-    status.flags.using_gps = (imuSampleTime_ms - lastPosPassTime) < 2000;
+    status.flags.using_gps = (imuSampleTime_ms - lastPosPassTime) < 4000;
 }
 
 // send an EKF_STATUS message to GCS