I have not designed Level 4 Spells and i don´t think i will.
But with my players, we researched spells for necromany, healing and shapechanging and recently, i developed homerules for it.
Here they are:
Researching New Spells:
You can expand your spell list and research new spells. Above all, this costs time, which you have to invest in studies and field research. You need 20 hours of research time for each rank you want your new one to have. You must then make a Knowledge check with the number of successes required equal to the rank of the spell. If you fail, you must research another 10 hours before you can reroll.
In addition, you have material expenses that you have to pay for. Researching a rank 1 spell costs 5 silver, a rank 2 spell costs 1 gold, and a rank 3 spell costs 2 gold.
Technically, you develop the spell together with your game master. Specify the name, rank, range, duration, ingredient, and description of the spell and effects along with it.