Will a 4% raising moon ruin a long exposure to shoot the stars?

as subject :/

4% won't affect the long exposures too much. If anything, the whole image may be slightly brighter but stars generally are too bright to be affected. That's for star trails.

If you're using the astrotracer then the contrast may be reduced very slightly by a 4% moon (and by slight I mean almost not at all).

Also factor in the fact that the moon won't be up for long at this stage.

I've shot at up to 50% illumination and in most cases I find that good post processing reduces the negative effects of the extra light.
Fainter objects may be washed out though.

Ha, tried last weekend since we happened to be in a fairly dark spot. Stars no, but milky way yes. So it depends how 'starry' you want the image to be I'd say. The more ambient light, the less stars are visible on the image. You can't counter it with longer exposures either, as the photographs simply come out too bright.

Apparently it's possible to take this photo in Dorset

https://www.facebook.com/DorsetScouserPhotography/photos/a.396682520400617.91942...

will I be able to shoot at with 4% moon or I shouldn't even try?

A 4% Moon will set shortly after the Sun does, so it will have no affect whatsoever. The Milky Way is brightest around midnight.