It would take over 200,000 people working 24/7 from birth till their 90th birthday at $25/hr to replace that $4T figure.
Well, good thing we have more than 200,000 people in the states then.
Currently, we have over 300 million people in the USA. Assuming at least 1/3 of them are employed it's a more manageable $40K each. Still not chump change, but it can certainly be paid off in less time than cradle-to-grave. Using your $25/hr number it'd take less than a year if they donated their entire salaries.
Assuming that our employees are instead paying a flat 20% tax, we'd need to wait for them to gross $200K before they'd have paid enough tax to cover this debt. For someone making $25/hr, that will take a little over 4 years working 40 hour weeks.
Admittedly, this plan would require (approx) a 20% increase in taxes for 4 years to pay off this debt *and* maintain our current tax-funded government plans. But while unpalatable, it's far more realistic.
Heard that the next Space Shuttle is supposed to carry several Guernsey cows? It's gonna be the herd shot 'round the world.