It is true that changing the size of the duct work closer to the end of the run is required for proper disrabution. Usually in order to keep the pressure the same all the way through the duct.
This is usually an engineered task done by the engineers at the time of design. No one should require their duct work changed unless you have been making adjustments to it.
Not sure why you would need to change the furnace from what was installed by the builder unless there was a problem with it. Like it was not set up properly or your set points were off. I assume you are stating that you have an electric furnace to heat your house. 30kw. Depending on the type of house you have, ranch, bungalow, two story, furnaces can take longer or shorter times to heat your house. The heating cycle should not be on all the time in order to heat your house. The AC could be on much of the time depending on your desidered set point (thermostat setting) outside air temperature and humidity. Because what you are stating is somewhat vauge and there are many variables. At first guess I would look at the control circut and thermostate to insure they are all working properly. Having a tech in trained to tune up your system may be all you need. A 4 ton unit cooling a 2400 sq ft house should be more then plenty in fact it could be to big a system. One should never over size your system as removing humidity is very important. If your system is over sized it will cool the house to quickly without removing humidity and you will end up with a cool damp house. Not good for your or your house. Have a tech tune up your system. I would call around first to different contrators getting their feedback on how they can help you before calling one one in.