Again, you are having to make some broad assumptions as to what the tax increases would do. If you look just at the percentage of increase in tax rates, and then slice that out of likely GDP, then you do get $4T. However, tax increases do not exist in a fishbowl. They will stimy GDP, so the pie will not be as large as projected. Tax rates change behavior.
With regard to "what-if's", in hindsight one can look at the cost of the Wars (about $1.5 T so far) and the tax cuts (about $3 T), and then make assumptions that our $14.5 T debt would only be $10.0 T right now. Possibly. But like so many things, we do not know what else would have gone differently. Had Al Qeada been able to better sustain itself in the last decade, we do not know if that would have yielded another major attack or two here, or in Europe. If and when we get the news bulletin that a major city has suffered a terrorist nuke, it will rock GDP by trillions.
If Medicare had been set up as a balanced program, what then ? As a percent of budget, entitlements are more than twice what they were 40 years ago, from something like 30% to 66%.