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.