In a sequel to the paper on small number primality detection by mental arithmetic.In this paper, we consider primality detection of four digit prime numbers, leading next to larger six digit and eight digit numbers, optionally scaled to arbitrary sized numbers. Several mental arithmetic techniques as mental arithmetic exercises from literature are cited, towards effective primality detection, by mental arithmetic only.The “large primes” mental arithmetic skill is developed both as a web based UI and as a UI based on slack, using the Wolfram Alpha nd Wolfram API , for primality testing and for Easter Eggs on prime numbers.Keywords: ASD, prime number determination, autistic savants, mathematical testing, Alexa Skills, learning and Cognition, Rabin-Miller test, Lucas test, fermat primes.