Imo, 4s is too much for that motor in a buggy. You'd be down around a 9 or 10 tooth pinion and turning well over 40k geared for 40 mph. That is, however, the motor I intend to run in my next buggy on 3s. With 3s and a 14th pinion, you'd still be geared for 40mph and the motor would run much cooler. My guesstimations are based on Losi 8 but most 1/8 buggies have relatively similar gearing. Hopefully some other folks will chime in and verify or negate my opinions.......

1512/2.5d(2000kv) would be my choice if your determined to run 4s. I had a 1515/1y in my 8; It was definitely overkill.