The grimoire the players found in Session 4 is "The Trifold Codex of the Planar Flame", a treatise on a
particular type of Elemental Fire magic. As a wizard's apprentice, you can't understand the spells inside, but you can possibly understand some of the theoretical discussions. For every 8 hours spent reading the book, make an INT-10% check. Success means you gained a greater theoretical understanding of Magic(can only succeed once).
You must now learn to apply that understanding by successfully performing the simplest ritual in the grimoire. For the ritual you need to gather solid manifestations of the 4 elements, and throw them together into a great fire, hot enough to melt them all. You must then perform the ritual, thus opening a temporary door to the Plane of Flame.
The material components you must gather in order to do this are:
Once you have completed this ritual, you have attained a greater applied magical ability and you may spend the XP on becoming a wizard.
Of course, one of those cheeky players asked where they are going to be able to find a fire hot enough to melt a diamond.
I pointed out that in Session 2 they were explicitly warned to "Stay away from the Volcano".
Bwa ha ha! Maybe he'll look for a Wizard to train him after all...