Where is the disconnect in the drive train?

Looking at your update - you didn't answer the question about what transmission you have. It's really simple - if you have an automatic transmission it's downshifting when you floor it - probably going into 3rd gear. 

If you have a manual transmission your statement about nothing being wrong with your car is incorrect - there is something wrong, your clutch is slipping. If that's the case get it fixed before it fails completely. 

if 2000 RPM is 60 in fifth gear, 4000 would indeed be 120 in fifth gear in a manual car with no problems. 

5-speed manual transmission? 

The clutch disc is worn to the point that the clutch pressure plate is now unable to grip it fully.  The torque of the engine is greater than the gripping strength of the clutch. This usually becomes apparent in high gear, then if you drive without repairing the problem, it gets worse.  It will start slipping in lower gears and get worse and worse.

As you make it slip it quickly wears the clutch disk to the point that it just won't work at all. 

Just understand something clearly, as you make the slippage occur, it makes great heat in the surface of the flywheel and clutch pressure plate.  The heat can create cracks and other problems in the metal.  Most mechanics get a remanufactured pressure plate and disc to renew the clutch, but if you injure the flywheel it can't be resurfaced and the flywheel must be replaced, which adds cost to the job.

The disc needs to be replaced.  The pressure plate should be replaced.  The flywheel should be resurfaced or replaced.  Hydraulic, self-adjusting clutches are sensitive to flywheel thickness being removed.  Throw-out bearing and pilot bushing/bearing should be replaced too. 

You can prolong the clutch a tiny bit by not driving aggressively.  In other words, not pressing the gas pedal strongly.

-General automotive mechanic since 1972 

Update: 

Your question made sense, for a manual transmission with a slipping clutch. 

Now you re-ask the question another way.  So first of all, I want to know.  DO YOU HAVE A MANUAL TRANSMISSION??? 

The "disconnect" in the driveline of a manual transmission is surely the "clutch". 

If 2000 rpm = 60 mph, then surely 4000 rpm is 120 mph, unless your clutch is slipping or your gauges are inaccurate. 

But if your gauges are accurate, and 4000 rpm gives you less than 120 mph, then something in your driveline is slipping.  If something is slipping and your car is not broken, then you do not have a manual transmission and a clutch! You must have an automatic transmission with a slipping torque converter. Normally, there is slippage in the torque converter, unless your transmission locks it up, so it cannot slip. 

Are you going in a big circle with your question, trying to have us point out that you have a slipping torque converter??  If so, then what kind of a car do you have with an automatic transmission that has five forward speeds?? 

ORRRRR, your automatic transmission is simply downshifting into a lower gear and the transmission gear ratio is changing. 

So please clarify your question more. 

You are asking us to please be kind?  What are you expecting?  Unkindness??  I'm now asking you to please be straightforward and direct.  Identify your car and its transmission.

If it's a manual transmission the clutch is bad and needs to be replaced. Make sure you resurface the flywheel while replacing the clutch.   If it's an automatic you need a rebuilt transmission.

There are computer controlled, hydraulically operated clutch packs in your transmission. With these clutches the computer can hold and release different parts of planetary gear sets to seamlessly shift between high and low ranges.

Fine AND dandy? wow

If it's an automatic, it's doing it's thing.  If it's a stick, it's impossible for the tach to instantly jump from 2000 to 4000 rpm while staying in the same gear - unless the clutch is shot and slipping badly.

The AUTOMATIC transmission DOWNSHIFTS to a lower gear AUTOMATICALLY, dumbass. A manual transmission cannot do that so you can only be driving an automatic. 

How automatic transmissions workhttps://driving.ca/auto-news/news/how-it-works-aut...

There's problem with the clutch - - get it replaced before it fails

If it’s an automatic transmission, it downshifted when you opened the throttle. If it’s a manual transmission, your clutch is slipping.