For a multi-hour exposure, mirror slap is essentially irrelevant. The vibration from the mirror only lasts a fraction of a second, so in a 4-hour exposure it contributes almost nothing to the final image.

Mirror slap is most noticeable in roughly the 1/30 second to 1–2 second range, where the vibration overlaps a significant part of the exposure. In very long exposures, other sources of movement matter much more—especially tripod shake, wind, or touching the camera.

Yes, adding weight to the tripod can help, especially in windy conditions. Hanging a bag from the center column is a common way to improve stability. Also use a cable release so you don’t shake the camera when triggering the shutter.

If you want to be extra careful, you can block the lens with a black card or hat while pressing the shutter, then remove it once vibrations have settled. That avoids recording any initial shake at all.

So: for star shots lasting hours, focus on tripod stability and hands-off triggering, not mirror slap.