I wouldn't suggest 4 of these under the seat.
First of all, you would need 1 cubic foot of internal space including driver displacement (which you can safely assume is at least .30 cu ft) So in essence, best scenario soundwise is 1.3 cu ft sealed w/o driver.
Secondly, these are 12", which will have tremendous cone area, allowing for quite a bit of air movement. The need for more than 1 or 2 will not justify cost versus return.
Lastly. the box to be built will have to be built to the seat for safety factor and asthetics. Therefore, eliminating the true use of the seat, and providing a muddy, muffled at best sound.
There are a few different forward firing designs built for that particular truck, but surely don't include the use of 4-12". The right design and application can yield tremendous bass response and sound reproduction, just do the research.
Possibly look into a downfiring center console style box for a great response as well, cabin gain is your friend.
Here are the airspace requirements posted by Memphis, this may help you decide a few things.