A 0.875" thickness coupon should have a minimun preheat and interpass temperature of 400 F.. For the thickness you listed, the 600 C. PWHT is right in line. But, the preheat temperature is very important in this case. Immediately after welding, the piece should be cooled to a temperature below the preheat temperature to a lower temperature where the transformation of austenite to martensite is completed. Then the piece should be immediately heated to the stress-relieving temperature where the martensite in the welded joint will become tempered and softened. After holding it at temperature for the specified time, the piece can be cooled to room temperature without the danger of cracking. You failed to mention exactly what filler metal you used. Corresponding fillers specifically for welding 4130 is listed in ASM specification 6457A.
CM