Frum repeated the same statistic in a give-and-take with Robert Reich on NPR yesterday evening.  That refers to 4.5% of GDP, not Federal spending.
Frum's argument was -- and I'm paraphrasing -- that 4.5% of our national income is a small price to pay for economic and political stability. He's right, of course. The biggest threat to the Western Military/Industrial machine is a scarcity of resources.