Just got myself a new flat bar road bike and would like to put a set of Ergons on it to give myself a more comfortable grip and the alternate grip positions the bar ends will provide me. The descriptions show the GP3 as a 3-finger and GP4 as full hand grip. Anyone tried both and can make a suggestion. I have used bar ends before on a previous bike and don't have an LBS in my area with either style. Suggestions?

Just a personal preference.
How big are your hands ?
How much bar end do you mind rising above your grips ?

My wife likes the smaller ones on her commuter, guess it is the gp3. She has smaller hands and wants less intrusive bar ends.
I run smaller bar ends on my flat bar hybrid/roadie bike than I like on my converted mtn commuter (biggest I could find then covered in old tube then grip tape) or steel monster cross commuter.

Another factor is:
How long are your rides/commutes ?
Longer I am on the bike, the more important multiple hand position becomes......
Then trekking bars, drops, or Jones H style makes more sense.

I normally wear a large or XL glove so I would say large hands. I don't really mind the bar ends sticking out, as long as they are not at the GP5 level where they are really out there. I ride maybe 20 miles maximum and prefer riding mostly in the standard flat bar position. Just figured somewhere to change positions once in a while to keep hands comfortable.

I can fold my Brompton bars down with the GR3. & not hit the wheel
(Back-to front about 4")

For my Bike Friday and its split -straight Bar,
I also have a set of GR5s they come around in front. J like,

gripping there and bending my elbows lets me lower my torso like going to the drops allows for Headwinds .
Much in the same way the figure 8 bend on trekking bars allow..

often I use the open relaxed palm laid on the Bar-end, /trekking bar side..

rather than holding on for dear life .. white knuckled.

I use the short ones on my mountain bike because I don't like the intrusive look of anything long. I find them long enough to grab on to when climbing etc and they give me another hand position when cruising along.

Rocking the GS2s (race version and same bar end size as the GP3) on my fat bike. I like 'em, no complaints here. I'm a big guy and these are plenty of bar end for me.
The angle is separately adjustable from the flat part of the grip which is nice for dialing them in just right.

I have the short version of the GP-2 on my mountain bike. I found that it wasn't wide enough, so I took an old Mongoose hand grip, cut it in half, and added half inboard of each GP-2. Now the fit/feel is correct for me. My hands are medium/large - I'd say average.

i've got GP5's on my hybrid and GP4's on my folder. i prefer the GP5's for the extra "way-out" hand position to get down and out of the nasty head winds along lake michigan, but the GP5's were too long to let my handle bar fold down properly on my folder, so i went with the the longest bar ends that would still work, the GP4.

in any event, i don't think you can go wrong with either the GP3 or the GP4 because 90% of why i love Ergon grips so damn much is the grip itself. best. grip. ever. i will never again ride a flatbar bike without ergon grips. i know that sounds awfully "shilly", but it's the truth.

as for the aesthetics of longer bar ends, i've actually always like the look of long horns on a flat bar (makes the bike look mean and aggressive), and i certainly prefer the look of long bar ends over trekking bars or other flat bar designs intended to offer more hand positions, but aesthetics are very much "to each their own".

Also, remember, if you use the Ergons w/ built in bar ends,
You can't use bar end mirrors like the mirrycle and others.
My biggest gripe with them.
I love the Ergon grips, though.