Lighter fork for Trek 4300

[Painkiller]

you probably have a zero-stack or semi-integrated headset so your only options would be a carbon fiber or aluminum fork. the least expensive would be aluminum then carbon from a lower end name like these

http://www.ebay.com/itm/Bicycle-2011-Mosso-MTB-Aluminium-7005-Rigid-Straight-Fork-Disc-Only-Black-White-/120988100723?_trksid=p4069.m2059&_trkparms=aid%3D111000%26algo%3DREC.CURRENT%26ao%3D1%26asc%3D27%26meid%3D309​6774899469490202%26pid%3D100039%26prg%3D1011%26rk%3D2%26sd%3D320946155716%26

http://www.ebay.com/itm/MOSSO-HM-Carbon-Fiber-Straight-MTB-Fork-Disc-only-/320946155716?_trksid=p4069.m2059&_trkparms=aid%3D111000%26algo%3DREC.CURRENT%26ao%3D1%26asc%3D27%26meid%3D309​6785414623172727%26pid%3D100039%26prg%3D1011%26rk%3D2%26sd%3D120988100723%26

http://www.ebay.com/itm/TRIGON-FULL-CARBON-RIGID-MTB-XC-Disc-FORK-26-480g-/230862904538?pt=Cycling_Parts_Accessories&hash=item35c0805cda

more common name brands would be quite higher. I have not used the Mosso brand, I have used the Trigon Carbon with V-brakes and found them to have flex @ the Bosses which would require a beefer to be used
but with the disc version that would not be an issue
I think the mosso carbon has an aluminum steerer tube and the trigon is all carbon
your bike was intended for a shock so it will be a give and take to go rigid with that style headset. if you do not have disc brakes I would go with the Mosso aluminum. If you do have discs just flip a coin , its up to the pocket book