According to my humble upinion the GPWS Mode 4 (A/B) callout "to low terrain" is a result of to high speed at to low RAD ALT not BARO ALT.
explaned mode 4(A/B)
4A - Unsafe Terrain Clearance With Landing Gear Not Down
This is the terrain clearance mode with the gear retracted, is armed after take off upon climbing through 700 ft radio altitude. When this envelope is penetrated at less than 0.35 mach the aural alert TOO LOW, GEAR is sounded. When the envelope is penetrated at more than 0.35 mach, the aural alert TOO LOW TERRAIN is sounded and the upper boundary of the envelope is increased to 1000 ft rad alt. The message is repeated until the flight condition has been corrected.
4B - Unsafe Terrain Clearance With The Flaps Not In Landing Position
This mode provides an alert when the gear is down and the flaps are not in landing position. If the envelope is penetrated at less than 0.28 mach with the flaps not in the landing position, the aural alert TOO LOW FLAPS is sounded. When the envelope is penetrated at more than 0.28 mach, the aural alert of TOO LOW TERRAIN is sounded and the upper boundary is increased to 1000 ft rad alt. The voice messages continue to occur until the flight condition has been corrected.
The TOO LOW GEAR alert take priority over TOO LOW FLAPS. The TOO LOW FLAPS and associated TOO LOW TERRAIN alert are inhibited with the flap inhibit switch when moved to the FLAP INHIBIT position
P.S trigger speeds may vary with aircraft type