IRA withdrawals are considered normal income. So they're taxed wherever income would be taxed if it came from any other source. Remember that tax rates are marginal; you don't "move" into another tax bracket, you just have some of your money taxed in that bracket.
So in your case, using the 2016 tax brackets and assuming a single filer, the first $9,275 would be taxed at 10%, the next $28,375 would be taxed at 15%, the next $53,500 would be taxed at 25%, the next $99,000 at 28%, etc., up to the last half or so which would be taxed at 33%.
On the other hand, given the same otherwise annual income ($8000), if you withdrew $200,000 on Dec. 31 and withdrew $200,000 on Jan 1 2017, almost all of it would fall into the 28% or less rates; you'd save 5% on almost $100,000 (or $5,000) and you'd save around another $10,000 on the other half (some at 10%, some at 15%, some at 25%).