The 4 star heroes available for purchase are the non-summoning 4 star heroes
Rewards of the Trial Edit
For every stage that the player completes he or she will be rewarded with credits and experience points. Every 5th stage contains a harder battle, and by winning those battles you will also gain gold and a hero shard.
For every 15 stages that the player completes, a check point will be created. This unlocks the rushing option, which means that you are allowed to skip the stages that you've previously completed the next time you enter the Trials. You will also gain the rewards of the previous stages when using the Rush option.