You just set up a globar variable named "score". Then you add these events and actions:
Player - On collision with another object (Coin):
- System: Add to variable (score): 1
- Coin: Destroy
System - Compare variable (score): = 4
- System: Create object (enemy)
Mind that though that if you didn't have any enemy created in the game before, it will throw an error (it's a limitation in HTML5). You'll have to create a dummy, empty layout (no event sheet needed for that one) and place an enemy there before you can create it. To put it simple: if the object wasn't added on any layout before, it can't be created.