Yes, your F4 shots will gneerally be sharper because lenses always
show their warts wide open.  f/4 on a good f/2.8 lens well, isn't wide
open.

And are you SURE you'll never wnat to shoot at f/2.8 and get really
shallow depth of field to really blur out the background?  Oh, I bet
you might.  :-)

Generally, yes.

Generally any lenses used wide open aren't as sharp as when they're
stopped down a notch or two. So the f/2.8 should be a tad sharper when
you close it down to f/4.

Once you get into the really good glass though, like the Canon L or
Nikon Pro lenses, the differences in optical quality wide open is often
too small to care about.

That's partly why I bought the Canon 70-200 f/4 L over the bigger and
heavier f/2.8 version. It's small and light enough to be carried
relatively easily, and with the Canon 17-40 f/4 L, two lenses cover most
of what I need in two reasonably priced and sized models.

The image quality of the f/4 is amazingly good, arguably the equal of
the f/2.8 model.

Additionally, the 2.8 lens will give you more light in the viewfinder
for focusing and composing, particularly when autofocusing in low light
or tough conditions or with a teleconverter.

A lens is usually at its peak sharpness 2 - 3 stops closed from its
widest aperture, so the f/2.8 lens will have a minor advantage in that,
er, light.

Manufacturers tend to "do their best" in the pro (eg: f/2.8) glass v.
the "very good" glass that they might limit to f/4 for cost reasons.

The above generalities, however, are imperceptible in most images you
make.  So if the extra shallow DOF is not of need, low light shooting is
not in the cards (not that 1 stop is all that important, a 1 stop of ISO
boost will suffice in mose cases) then the less expensive glass will do
you you good and not be as heavy to carry.

Cheers,
Alan

You might want to upgrade to f/2.8 zooms later. The new Canon f/4 IS
zooms are not that much cheaper. They are more for people who want to
save weight, than for people who want to save money. If money is a
concern and you don't want to lose too much money when you upgrade to
f/2.8 zooms, you can also consider f/3.5-5.6 variable aperture zooms
for now. The new computer designs are much better, e.g., Canon
70-300/4-5.6 IS, Nikon 55-200/4-5.6, ..., especially if you plan to
stop down to f/8.

http://digitcamera.tripod.com/#slr

most lenses are better at one stop down than wide open and hit their best at
2 stops down.

f/4 lenses tend to be made for the consumer market and are cheaper in more
than just price.

you can never be too thin too rich nor have a lens that is too fast.
actually having that extra stop really means a lot since much of my portrait
work is window light, twilight and open shade, also with flash and ambient
light blending where you select an f/stop to control the flash with a slow
enough shutter speed to allow the ambient light to come up to one stop under
the key light exposure and WOW, the image gets great depth and detail.
works great for weddings in churches, the bride isn't a washed out ghost in
a coal mine.