I'm trying to do an adjusting entry with beginning balance in Inventory at $4,000,000 and I need a value in Ending Inventory of $3,550,000. Do I need to credit Inventory $450,000 and debit Purchases by the same amount?

Your debit will be to Cost of Goods Sold.

Certainly credit inventory but it is more likely your debit will be to cost of goods sold of course there is a possibility it is stock shrinkage or work in progress providing a context is useful