find 2 prime numbers that if multiplied would generate a 400 digit number

Question

I'm not sure what you want. 400 is a three-digit number, and 4000 is a four-digit number. I don't think you want a "400 digit number."

Assuming you want a number with a value over 400, this site will be helpful:

http://mathforum.org/dr.math/faq/faq.prime.num.html

I hope this helps. Thanks for asking.