The Orange Box

ok this isnt really that hard but u kinda need to be paying attention.

1.After getting back your car from the mechanic. drive through the sludge then turn left. it will be in the van with a car wedge up on it.

2.After passing a water tower on the left hand side you will come to a building with a van that has run into the side of a building. go into the bluiding and there will be a legde you can not get to pull the big piece of metal leaving on it onto the floor then throw a grende under it the launch you up to the legde to flip switch.(you should also recieve the "Gordon Propelled Rocket" achievement)

3.Drive straight across the road into the boxcar yard and you will see a shed missing part of its roof in the back. go inside and you will see a saw blade in the floor pull it up to uncover the cashe.

4.After going through the tunnel and going up a curve hill there will be a tree log to your left. go behind it and there will be a small cave. go inside and there's your 4th cashe.

5.Afer fighting off the combine and powering down the fence to free your car. you will jump off a ramp and drive down another curve hill it will be on your right side behind some rocks. there will be an arrow pointing up. there are some rocks there to break the rope that has the cashe hanging on the tree. (i used the secondary fire on my smg to get it down)

The other solution above is correct, but if you prefer a video guide here you go:
(great guide, i used it.)
Please note! i'm not the maker of this video, credits go to: AlfaWAR