my gen 4 iPod doesn't charge, the usb cable is good(works on other apple products) What should i do?

This is a "gen 4 iPod" (very old iPod) or a 4th gen iPod touch (shown in profile)?

If it's a touch, and depending on where that cable is connected, it's possible that the connection does not supply enough power for the touch. If your "other apple products" are smaller iPod, it may be sufficient for those devices.

With that additional information, I would say that your iPod touch should also charge from the same connection.

If you have not already, be sure to try restarting and resetting the iPod touch

http://support.apple.com/kb/HT1430

If that does not help, the problem may be a hardware problem on your iPod touch.

You can also try a Restore, which erases the iPod, re-installs the latest software, and sets it to default settings. This document provides instructions.

http://support.apple.com/kb/ht1414

A Restore will only fix the problem if a software glitch on the iPod is the cause.

If that does not help, you'll probably need to have it checked by a tech. If you have an Apple Store nearby, you can make a reservation at the Genius Bar (using the store's web page) and take it there.

http://www.apple.com/retail/

NOTE: If the iPod is less than one year old, it is under warranty. If you bought AppleCare for it, the coverage extends to a total of two years.

What normally happens is most people take showers and listen to thier ipods or live around many sources of humidity. Moisture corrodes the 30 pin connector on the ipod touch. Clean and inspect the ipod touch's connection pins to resolve most charging/ sync issues.. The pins could also be dirty, bent or broken. If dirty: Take 91% rubbing alcohol, dip a tooth brush into it, shake the excessive alcohol off leaving the brush damp(don't use to much alcohol, it should not be dripping). Careful!!! The Pins are Very Fragile! Gently brush the connector pins, long ways, left to right in a few second bursts, inspecting the pins as you go... If you have an eye loop or magnifying glass you can see inside the connector easily and see if the pins look dirty or bent or broken. If they are bent you have to VERY GENTLY stand them back up. If they are bent very far down, chances are you will snap them off trying to straighten them back up. If they are not severely bent over, use a very small flat head precision screw driver to bend the pin back parallel with the good pins. Pease note: certain pins are used by the wire for different things such as sync or charging... If your pins are broken off, do a search and find out what the individual pins on the ipod 30 pin connector do to see what is affected. Worst case is your pins are broken off. U can attempt to solder the broken pins back on without opening the ipod touch, which is super difficult to do, or you can open the touch and repair/replace to 30 pin connector entirely. U will need to do some micro soldering to say to least! Don't attempt unless you are confident in your soldering skills and research how to open an ipod touch? You will need some specific tools and watch out for floating LINT! Get some canned air ;) use a small 1/2 second bursts from 4-6" away because it will steam up you touch if used for to long. Good luck!