I'm using cards I got i the anniversary special and the kickstarter from way back when.

I'm so regretting not getting in to this earlier. My first ever reward was a beta pack and the very next day it was switched to reward cards.

You aren't stuck. Combine the other 3 cards first. Then add the 4th and the new card will be playable. 😁

Thanks for the tip, but I guess I did it the wrong way around so in this case.. I'm screwed!