One point that no-one seems to be pointing out is the fact that your Tc will also take away light gathering abilities also, and no matter what, it will degrade the image slightly.Try taking a shot of small birds under canopy of trees in shade with such slow lenses and you will be very disappointed.
On your 4-5.6 lens a 1.4X Tc will make it a 5.6-7.84 aperature and you will only have manual focus on most bodies on any aperature over 5.6 with a TC attached.
I would only recommend using a Tc with the highest quality glass. (speaking from experience here).We use both 1.4x and 2x Tc on both our super-telephoto lenses but they are the 300 f2.8L, and the 500 f4L glass which are at the top of the heap in IQ.
I would recommend you take a look here, http://www.slrgear.com/reviews/index.php, real reviews from owners of all lenses.
Hope this helps.