I don't know if you've seen it (it seems to get way more hits than I'd expect it to) but I did a final year project on Prime numbers and their generation (with source code for Pari gp) that you might be interested in.

If I remember correctly I was able to generate probable primes of 200 digits very rapidly with the code I provided. So you should be able to get lots of 400 composite numbers if you want to. You'll have to download Pari gp though, but that shouldn't be too hard. ;)

Have a look over at: http://perltraining.com.au/~jarich/Primes/index.html

I hope that helps.

jarich.