First find his current travel time 600 miles / 50 miles per hour = <<600/50=12>>12 hours.
12 hours - 4 hours = <<12-4=8>>8 hours desired travel time
600 miles / 8 hours = <<600/8=75>>75 miles per hour.
75 miles per hour - 50 miles per hour = <<75-50=25>>25 miles per hour faster
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