How old are these plants when they are 4 inches tall and what size pot are you using (I suspect Rootbound if they are all consistently dying at the same age/height).

You said you don't use enough perlite. Promix is a peat based mix. It retains water very well. Too well. When using promix I mix at least 33% perlite.

Do you have pictures of the problem and can you fill out the troubleshooting form.

Those two things combined could have your problem solved much sooner. :thumbsup:

Also does it really smell like pine? Or more like Ammonia?