Here's a step-by-step breakdown of the procedure. This is basically the same regardless of whether it's in Close Combat or Shooting.
- Select an Eligible Unit.
- Declare targets for all weapons before making any hit rolls (important in units with mixed weaponry)
- Pick one weapon type (in some situations you may need to attack with each weapon one at a time, but usually you can save time by rolling groups at once) and make your to hit rolls.
- For every Hit make a wound roll. Again sometimes you'll need to roll the wound rolls one at a time, but usually you can group them.
- For every wound your opponent needs to pick a model and make a save. If the AP (Armour Penetration) of your weapon makes their regular save worse than their Invulnerable save they'll use the Invulnerable. For a unit w/ multi-wound models, if a model already has a wound it must be picked first for this step. If the damage of the weapon is greater than the wounds of the picked model and that model fails the save it is removed and a new model is chosen for the next wound. Keep going until all the wounds are applied.
- Sometimes a unit will have an effect (often from another unit, or a psychic power) that lets it ignore damage (such as Disgustingly Resilient). Often this will be referred to as "Feel No Pain" as there was a universal rule with that name in previous editions. If so, for each point of damage it takes from failed wounds (or Mortal Wounds) it gets to make a FNP roll. If successful that damage is ignore. However note that a roll is made for each point of damage. So if a 2 wound model failed its save from a 3 Damage wound, it would need to make at least 2 of its FNP rolls to avoid dying.
If you've got mixed save units w/ FNP and multi-damage damage wounds you'll need to go through the process one at a time as things can get complicated. Once you've played a few games you'll figure out when and where you can speed things up by rolling multiple saves/FNP at once.
Note that mortal wounds ignore saves (regular and invulnerable) completely but don't usually ignore Feel No Pain type effects...
In the specific example of 8 hits > 4 wounds, the orks would make 4 saving throws (needing 6s). If you had a single ork with an invulnerable save, you could choose to roll against its Invulnerable, but if it fails that ork in particular would die. Since they're all single wound models, the extra damage is wasted.