4. Warehouses A and B originally each stored whole bags of grain. If 90 bags are transferred from Warehouse A to Warehouse B, then the number of bags in Warehouse B will be twice that in Warehouse A. If a certain number of bags are transferred from Warehouse B to Warehouse A, then the number of bags in Warehouse A will be 6 times that in Warehouse B. How many bags of grain did Warehouse A originally have at least? $\qquad$ 

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【Answer】153
【Analysis】The total amount of grain in the two warehouses is constant. After two adjustments, the grain in Warehouse A accounts for $\frac{1}{3}$ and $\frac{6}{7}$ of the total, respectively. Let the total number of grain bags be $21a$, then the number of bags in Warehouse A is $\frac{1}{3} \times 21a + 90 = 7a + 90$. After transferring a certain number of bags from Warehouse B to Warehouse A, the quantity in Warehouse A becomes $\frac{6}{7} \times 21a = 18a$. Therefore, $7a + 90 \leqslant 18a \Rightarrow a \geqslant 9 \Rightarrow (7a + 90)_{\min} = 153$