The only structure that you will be able to build the Mammoth Mk IV from is the deployed defensive Crawler. As such it does not replace the TD Mammoth tank as the main high tier mass production vehicle, instead it is considered a 'luxury' addition.
The other thing that you should know is that the only way to upgrade this Mammoth tank is to fully upgrade your defensive Crawler. Whether you consider this worth your time and money is up to you, maybe this information will help you decide.
The basic Mammoth Mk IV has slightly better health then the TD Mammoth, and also a slightly better self repair (it works even when the Mammoth is taking damage). The rocket pods however have a much longer reload then the TD Mammoth, and are not close to being as effective against infantry.
"You cannot run!"
Since by the time you can build this Mammoth tank, you would also be able to upgrade your TD Mammoth with sonic grenades, deciding to build this unt might not be as simple as you think.
So finally we get to something really different.
This laser can only target ground units and structures, and has two different uses. Vs infantry it operates as a dazzle laser. The effect of the laser on infantry causes them to go prone, and greatly reduces the speed and range at which infantry attack. Finally it will also do a small amount of damage to infantry. This is more useful against infantry then you might think.
Vs vehicles and structures, it works as a target laser that allows all of your units to do slightly more damage to the targeted unit (10% more). Unlike the dazzle laser, the effect of the target laser stacks up to 5 times (ie up to 50% more damage to a targeted unit).
Obviously the target laser is the difference, and vs the Scrin and Nod late game units, a few Mk IV will be very handy if used correctly.
One of the major irritations that some had with this mod, was the removal of vehicle based rail attacks. I just never liked EA's implementation.
Finally GDI can have vehicle based rail attacks, but see how awkward it is to get to!
To get the beast Mammoth tank, not only do you have to have a defensive Crawler, but that Crawler has to be upgraded to its maximum level! (Tech level III).
Once you have done this all Mk IV's will have not only have rail guns, but will also have double the rocket ammo, a true beast of a vehicle, if you factor in the extra health, better repair, and target\dazzle laser.
The final note, if you remember from the Crawler news article, that the final upgrade boost the production speed of your Crawler? This means that you will be able to pump out rail Mammoths at twice the speed!
I feel bad for your opponent!