Know anybody at the local fire station that has a water truck?  Maybe
they could deliver some to you.

Consider this, the M149, Trailer, Water, 400 gallon, in slang a water
buffalo
was designed to be pulled behind a 2 1/2 ton truck or larger. It was
forbidden to pull it behind M880 5/4 ton truck because when the water
started sloshing around it would push the truck all over the place.
http://www.olive-drab.com/idphoto/id_photos_m149.php

water tanks are always smooth bore, no baffles, so that 4150 pounds is
constantly moving, and when you try to stop it sloshes foreword pushing you
foreword.  Too fast in a turn and your going over.  I wouldn't even trust an
F-350 with that kind of a load.  Nor would I trust the bed of a pick up
truck to provide a secure tie down, that kind of weight needs to be
connected directly to a very stout frame.  Remember the 4150 lbs is static
weight.

Whitelightning

A lot of people in my area haul those round green water tanks that have a
flat bottom and domed top in 3/4 ton trucks all the time.  Just make sure it
is totally full so you don't get the sloshing, and slow down som for sharp
corners.  I've seen them hauled on sturdy trailers, too.

I would suggest the F-250 diesel. It has the torque to move the load easily,
and the load capacity to repeatedly handle a load weight of that size
without having to maximize the output on the suspension system. An F-150
while ample would end up sagging in the rearend and require spring
replacement far sooner than the F-250. Here is a chart of towing capacities,
keep in mind the actual dead weight capacity vs the towing capacity is about
50-75% of the towing capacity. You need to keep in mind that if you are
hauling an 8,800lb trailer that the tongue weight is usually 10-15% of the
weight with 5th-wheels being about 25% . So if an 8,800lb trailer causes the
F-150 springs to sag drastically, imagine what 4,000lbs of water right over
the axle will do. You will also notice that the highest towing capacity in
the chart is the diesel engine and ranges from 10,000-14,500lbs with a
manual transmission.

http://www.trailerlife.com/downloads/00towingguide.pdf

PWK

I've driven forestry trucks for a fire department with 125 - 150 gallon
tanks with baffles.  We use a F-350.  You're talking 3 to 4 times the
weight.  If you put this on a heavy enough trailer, it might work, but
you're going to have 2 tons moving all over the place (called 'surge').  
Two tons pushing on any pickup truck will cause the truck to move,
sometimes in ways you can't anticipate.

I have a CDL with tank endorsement, and have hauled over 5,000 gallons.  
The more fluid you have, the more push you have on a truck.  When you
have a surge slamming against the front of the tank when you're trying
to stop your vehicle, and running out of room, you learn what 'out of
control' means.  Without baffles, I wouldn't run that amount of water
with less than a six-wheeler, probably a two ton.  

I'd think about picking up a used tanker, especially from a fire
department.  Our department just sold one with a 1,000 gallon tank and
less than a 100,000 miles for $5,000.  Do not buy an old oil tanker, as
the surge will rip the tank to pieces.

Jim Clark-Dawe

One that will hold a 2.5 ton payload, neither of those you mentioned can.
Look at an F-450 at minimum, or buy a trailer to mount the tank to.