Knowing the details of the winch would help but yah, you're fine with 4 gauge at those lengths and for the durations the winch will be used.
By the way, don't know what kinds of wires you are having "made" but you can just buy the wire by the feet from all sorts of places like car audio shops for example. The other option that sometimes ends up being very cost effective is to buy battery jumper cables (good ones in the spec you need) and then cut them up to size.
As far as splicing thick existing cables, you can crimp terminals on them and bolt them together or interleave them nicely and use small hose clamps to tighten things down. Heat shrink tubing is a pretty good way to insulate or you can do the electrical tape and then just throw some zip ties on for good measure. I have this roll of rubber tape that bonds to itself and makes a pretty good waterproof seal and that's another option.
By the way, don't know what kinds of wires you are having "made" but you can just buy the wire by the feet from all sorts of places like car audio shops for example. The other option that sometimes ends up being very cost effective is to buy battery jumper cables (good ones in the spec you need) and then cut them up to size.
As far as splicing thick existing cables, you can crimp terminals on them and bolt them together or interleave them nicely and use small hose clamps to tighten things down. Heat shrink tubing is a pretty good way to insulate or you can do the electrical tape and then just throw some zip ties on for good measure. I have this roll of rubber tape that bonds to itself and makes a pretty good waterproof seal and that's another option.