Glock 17
 

My glock is shooting about 4'' to high do I need to get a taller front sight if so what size shoult I get

What kind of sights? What ammo? What distance? What sight picture are you using (6 o'clock hold or dead center)?

Glock 17
 

I am useing xtp bullets at about 25 yards and I have ajustable sights on the back I have set them low as they will go

Factory loads?

It sounds like you may indeed need a taller sight if that is the case. Also most Glocks need a 6 o'clock hold on the black of a 25 yd bullseye to hit center at 25 yds.

Glock 17
 

How much higher would go on the front sight , I am pointing at the center

I stole a couple of numbers from http://www.gunrightsmedia.com/showthread.php?408305-Glock-19-17-34-Sight-Radius-effects

If your sight radius is 6.5" and you are shooting 4" high at 25 yards, then.

h= required increase in heigth.

25 yards is 12 inches/foot & 75 feet or 900 inches

Now we are happy 'cause everything is inches, and lord knows we don't live in a foreign country.

So from the diagram below we get h= (6.5" * 4")/900
.028888 isn't much, call it 3 hundreths or less than 1/32" increase in heigth.

Try a 6 o'clock hold-

Works fine for standar bull targets, not so good for shooting shotshell husk or charcoal briquettes. or a turkey head.

I'm all for the POA/POI at 25, then applying kentucky windage from that point.

But, after the math says t's less than 1/32 of an inch, I would "hold low" also, I am not so sure that he would want to go through replacing, testing, filling, testing, filing, testing, and finishg, for 1/32"

I would suggest trying different ammo to see if it will get you closer to your point of aim. About the best group I have ever gotten out of a G-17 at 25 yards is 2-1/2" out of a machine rest, so you might not be too far off. If you are going to fool with the sights I would suggest picking up an adjustable set of Glock sights off EBay. You can usually find some new takeoffs for around $20. Changing out the fixed sights with adjustable will most definitely solve your problem.

Lighter bullets will impact lower than heavier ones.