Most likely they were breeding as previously stated. I would drop the wafers only once a week & I would drop 2 in different areas of the tank. Also, I suggest feeding veggies such as Kale, cucumber, etc. Once or twice a week in place of the algae wafers. 4 Mystery snails in a 5g is quite a lot, it is recommended to have 2.5g per snail, if I remember correctly so you're double that. You will be quite over run with snails if they are breeding & you choose to let the eggs hatch. Keep in mind they can lay 100's of eggs at one time.