Any Suggestions Extending 4 Guage Wire?

I need to extend a bunch of 4 guage wires to relocate my battery.  Does anyone have any suggestions on the best way (other than replacing the entire cable) to extend a 4 guage wire?  Any kind of union, etc.  Thanks.

They sell copper? sleeves that you can use to slide both ends into and solder it together.

2 ring terminals and bolt them together and tape or shrink wrap the joint.

Depending on how everything is setup, if you are saying a bunch of them, I might suggest running them to a distribution block and then run 0 gauge back to where you need it.  Really depends on how many you have to extend and how far you need to extend them.  As far as I'm aware there really isn't a Union persay available.  You could run an inline fuse terminal (the glass tube style, with twist ends), but thatt would get clunky real fast and could cause issues locating a problem if you ever did blow a fuse.

For power you can easily put a circuit breaker in to help protect the new length.  For the negative side they do make connectors, have to goto a electric supply place for those.If you do not have any luck finding them I can ask my dad if I can get some from his work. 

I have seen something that would work at Home Depot and Lowes.It is in the electrical section. Basically it is an aluminum bar with holes milled into it for the wire to go, and set screws that clamp them down( basically like most distro blocks and fuse holders ). It comes with a heat shrink sleave to put on after the set screws are tightened. Of course just using a fuse holder of circuit breaker would work fine as well as adding protection to the wire. As far as the negative cable, you should be able to simply ground it near the battery to the frame( unless for some reason the body/frame are a poor ground...can be the case with some unibody cars I hear ).

i have personally used the aluminum union previously stated and it worked great for relocating my buddies harness...also if you ask around at stereo shops you can actually find 4 gauge butt connectors typically yellow colored just make sure you have bad ass crimpers or use a vice to ensure a lasting crimp....idk if it will benefit u at all or not but we also got some pvc tubing that screws together and has a flange on it for all the wiring to come from under the bed into the battery box without having to find a big rubber grommet.

i agree with that mike. they say you have to run thicker back and not smaller or the same. so id run 0gauge. when i was looking for terminals for my hydros i came accrooss some big distribution blocks that look like a stinger brand.  its not stinger.  i searched google and they came up some were.  there 0 guges to 2 gauge

the yellow ones dont work or 4ga without trimming some of the strands.  they have 4ga unions with set screws on both ends to extend 4ga. i think stinger and tsunami make them. i didnt see them often but i know for certain ive used them b4. its a barrel with a set screw at each end. hit up a stereo shop, they can order them for you. prolly not cheap though. if you cab find a copper sleeve(make one) that just fits the wire then solder the wire ends in the sleeve and wrap with some of the thick electrical tape (the kind that sticks to itself dont remember the name) or some thick heat shrink it will make a nice clean and compact union.