Not necessary to change anything or use Horde rules either, if you don´t want to…

If you mob is large enough that is, 15 humans without equipment against 4 well equipped PCs should do nothing really and if the game mechanics work that way it should be okay.

Now, if we are talking about each PC being mobbed by 30 human NPCs at the same time that is a different story. Just to make the math easier, I rule that the NPCs have a Strength of 30 and Weapon Skill of 35, you will see why shortly.

Now, your problem is that PCs can reduce 11 damage (4x2 armor + 3 TB average) if I understand. Now, you have 30 guys attacking one PC, they get +30 WS for outnumbering, or more (up to +60) depending on the bonus limits you are using. +30 WS means 2/3 of the NPCs will hit, that would be 20. Assuming that those guys cause 1-10 damage with no Righteous Fury, you can go with the statistics average, and assume 2/20 NPCs would roll 1 damage, another 2/20 would roll 2 damage, and so on until 10. Now 10+3 is 2 points of damage x2 NPCs, plus 10+2 for another 2 guys. That´s like 6 damage on the first round (might drop later depending on the amount of NPCs), and it will take quite a few rounds to kill so many NPCs. Consider players cannot dodge because they have no room to do so… Adjust that math to how much damage you wish to cause them and you are set (my math of 6 damage per round very likely means TPK in just few rounds)…

If you don´t wish to pump WS so high, just don´t limit melee outnumber bonus at +30, or do a different math (the above suggestion was just to use easy to explain numbers).