4000 Sparks. Now what?

If you have to use it, use it to craft your needs

if you already have a good decks, just save it for future event cards. 4000 sparks / 750 gems is little. Save it

How you spend what you have reflects the kind of player you are.  For me, the most important use of my gems (once I realized it was an option) was buying the Heroes @ 750 gems a pop.  After unlocking all the Heroes (none gained through lucky pack draws), I started using my gems on the multipacks to get more cards.  I prefer to use sparks to get event cards I missed out on, but sometimes I'll also go for a card I like or don't have any copies of.

The only legendary that I've ever crafted and I absolutely love is the Trickster. The fact that you have a potential 12 damage burst from hand is just nuts. 

You can use it to clear annoying plants and get a zombie on the board at the same time as well.

If you like the Professor, you may also consider crafting the Gargantuar's feast. It's SUPER difficult for the plant hero to remove all 3 gargantuars without taking some serious damage.

(If you play a Gentleman Zombie on turn 9 and it survives, you can play the feast on turn 10 since it gives you the 2 extra brains you need).