I realized what you meant a bit late but it seems like lots of answers. I just posted a thread asking about mastering foods and the gifts at each level. Someone was thoughtful enough to point me to the correct point in the Game Guide which I overlooked
It may not be feasible to reach level 4 but according to the Guide both Levels 2 & 4 give you gems. Maybe you can at least reach level 2... 3 gems are better than none.
Here is the Guide if it helps...
Mastering Recipes
You can earn the following rewards upon completion of each mastery level of all recipes. The recipes in all appliances generally give 1 gem for completion of mastery levels 2 and 4 respectively. However some appliances have recipes which reward you with 2 gems instead of 1. (e.g. Colonial Oven, Yogurt Chiller)
How Many Cooking Sessions To Master Each Recipe?
..Mastery Level Servings Rewards
1 (Apprentice) 8 2,000 coins, 20 xps
2 (Line Cook) 23 1 gem, 50 xps
3 (Sous Chef) 43 4,000 coins, 100 xps
4 (Pastry Chef) 83 1 gem, 200 xps