Vector4 tmp = (inverse(get_view_frustum() * get_camera_matrix()) * screen_click; //take the inverse of the camera matrix as well
tmp /= tmp.w; //homogeneous coordinate "normalize" (different to typical normalization), needed with perspective projection or non-linear depth
Vector3 ray_direction = normalize(Vector3(tmp.x, tmp.y, tmp.z)); //make sure to normalize just the direction without w
[EDIT]
A more lengthy and similar post is here: https://stackoverflow.com/a/20143963/1888983
If you only have matrices, a start point and point in the ray direction should be used. It's common to use points on the near and far plane for this (an advantage is if you only want the ray to intersect things that are visible). That is,
(x, y, -1, 1) to (x, y, 1, 1)
These points are in normalized device coordinates (NDC, a -1 to 1 cube that is your viewing volume). All you need to do is move both points all the way to world space and normalize...
ndcPoint4 = /* from above */;
eyespacePoint4 = inverseProjectionMatrix * ndcPoint4;
worldSpacePoint4 = inverseCameraMatrix * eyespacePoint4;
worldSpacePoint3 = worldSpacePoint4.xyz / worldSpacePoint4.w;
//alternatively, with combined matrices
worldToClipMatrix = projectionMatrix * cameraMatrix; //called "clip" space before normalization
clipToWorldMatrix = inverse(worldToClipMatrix);
worldSpacePoint4 = clipToWorldMatrix * ndcPoint4;
worldSpacePoint3 = worldSpacePoint4.xyz / worldSpacePoint4.w;
//then for the ray, after transforming both start/end points
rayStart = worldPointOnNearPlane;
rayEnd = worldPointOnFarPlane;
rayDir = rayEnd - rayStart;
If you have the camera's world space position, you can drop either start or end point since all rays pass through the camera's origin.