I feel like 4 years would be a little bit too much. At 34 years old, 4 years later he would be 38 and yes he could play "somewhere" but as we have seen even with Ronaldo, the guy with the most goals in history of football, when you get to that age it is not easy to continue playing. Ibrahimovic may have done it until he is 41, but he plays for Milan and he was never their "star" player anymore, plus Milan is not Barcelona, there is a difference.
I think Lewandowski can play until 38, but he can't play at Barcelona until 38, he could play 2 full seasons there, and then a third season where things will show that he is not enough anymore, and be gone before fourth season.
I have to admit that you are probably right. No matter how much I want Lewandowski to continue with Barcelona, age is a very big factor for consideration. But I still believe that he maybe able to stretch for 3 seasons without a visible degradation in form. Playing for Barca at the age of 38 will be tough. But there is a lot of demand for players such as Lewandowski, and similar to what Cristiano Ronaldo did with Al-Nassr, he can sign a huge deal with some of the cash-rich Arab clubs and easily earn a few hundred million USD before he retires.