You talking about headlight or driving light? I am assuming running light as that is H4.
Changing out the headlight is not for the faint of heart. And not very easy.
I converted the headlight on my Venture to a HD headlight that had the old 3 bars inside the light look. I had to basically remake the entire headlight nacelle and frame. A very good thing the fairing hid everything because it was not pretty. You would have to do the same thing on the Voyager. It is not just swapping out the halogen bulb out with a LED one. I have not seen a non projection LED headlight that can actually work for a headlight replacement yet.
I have been thinking about doing it again with the Voyager with the new HD LED system. I have seen them as low as 800 bucks.
You will have to throw everything that is headlight related in the Voyager fairing away and build the frame and everything from scratch. But I will try using a HD headlight can from a non fairing HD that the LED headlight will fit and all of that will be hidden inside the fairing.
Now... I have also put super bright LED running lights in it. They are very bright to oncoming traffic with the light bluish tint the LEDs make, but do not do much for you to see down the road. Because the light coming off the LED assembly is not focused forward like the halagen light bulb is. But that is what I want. In daylight you can see the LED running lights before you see the stock headlight even with it on bright.
Look in this thread. I have pics of the LED lights I got. DO NOT get the ones I tried at first. Scroll down and see the ones I have in it now....
http://www.vulcanforums.com/forums/showthread.php?t=154122