How to go from Vrrrrrrrrooooommmm to Vrroomm Vrroomm?
The first answerer is off, but the second is headed in the right direction. It all has to do with what sort of loads the engine has to bear against and how well it can breathe.
The most direct reason, and where I suggest you start for the biggest results, is the mass of the flywheel. Addressing your experiences, the Mustang was a muscle car, and was built accordingly; the engine has less initial resistance (i.e. its flywheel is lighter), translating to quicker response and more power to the wheels. The Maxima is a "performance sedan," meaning it will behave nicely while still having a bit of get-up-and-go (read: heavier flywheel, but more horsepower). So, there is the comparison: the Maxima has more overall power, but a greater percentage of the Mustang's power actually gets to the wheels, making them feel about the same in acceleration.
The reason you found that the four-cylinder engines didn't rev-up as fast is because, you guessed it, the flywheel. Automakers do this to make the vehicles more fuel-efficient: if the engine is already spinning from the residual inertia stored in the rotation of the flywheel, then less energy (fuel) has to be spent maintaining or raising this speed.
To answer your question, I recommend you get an aftermarket aluminum flywheel, or just have the stock one drilled for lightening holes (I do not suggest altering the thickness, for you may see some adverse clutch response). As indicated above, this is also a great way to increase (usable) horsepower. Other improvements (having significantly less noticable effect) would be done is this order:
larger throttle body/carburetor(s)
larger exhuast (the best being mandrel-bent pipes) *NOTE* you can have too large of pipes, so do some research on you engine
After adding/changing these things the only thing left would be to remove the compressor for your AC system, but I figure one should not be without such creature-comforts.
If this were going to be a drag car, you could also switch from power steering to rack and pinion, eliminating the power steering pump.
All of these suggestions are *just* to increase rev response. You could have all sorts of engine machining done which would raise the dispacement and indirectly increase rev response. Adding a tubor/supercharger will add load to the engine, but of course the added power would compensate for this.
As a warning, reducing the flywheel mass will have a drastic effect on the driveability of the car and will take getting used to. Find someone who has the same or similar vehicle, but set up for racing, so you can get a feel for the other side of the spectrum. Aluminum flywheels cost several hundred dollars (new), but if you go the drilling route you can slowly decrease the wieght untill you find what you like (though this is *very* labor-intensive).
Have fun wrenching!