Shemittah ends in Tishri. In Yovel, after the 3rd temple is built, the food grown in the shemittah year would cover food for eating in the Yovel?
ב"ה Shalom The torah in Vayikra 25:22 speaks of the divine promise of blessing the land for three years, meaning the produce from the sixth year would suffice for 3 years. However, indeed when the Shemitta year and the Yovel are in consecutive years, the blessing would be for 4 years as the commentators point out ad loc. All the best