Yes, that's right 4 bms if the 2 boys die and the 2 grotz survive.
if the grotz die then you get only 2 bms (assuming there are other non grot units remaining)

im most cases i suspect , you'll be hoping for low rolls for those grot saves. If there is a threat of disrupt on the table, you might consider keeping the grits out of terrain

Thinking about it you'd allocate them to grots anyway to give yourself the chance of taking no markers at all!!

Anyway, the point is allocation does matter and you need to save for special hits/different units separately...

But causing the extra BM's are what disrupt is about.
Formations attacked by them should take more BM's than if they were attacked by just normal hits, it's just makes it harder for players to use "gamey" allocation of hits to try and minimize those BM's.

Right, I'm confused now...

How should it be played then, I don't want to be gamey, I just want to do it right.

No. Well generally that's correct, but in the case of disrupt you wouldn't. if your grotz had a chance to save then you wouldn't want to allocate to them, because when they do save they'd get a bm each.

In practical terms though, the choice is out of your hands since disrupt is pretty much universally combined with barrage - (mainly elder and imperial guard artillery)

Overall, in general, you might say that the disrupt rule increases the number of BMs. But if you combine it with the grot rule where grots are in cover then, weirdly, what you end up with a complete reversal of the normal situation. A kind of flip over of the normal rules:

die= 1 bm, save = 0 bm becomes save =1 bm, die = 0 bm

quick question on procedure determining bms on grotz btw.
Should you, strictly speaking, make saves on non grot units first, to see if there are any non grot units left and hence to determine if the grotz qualify for the grot rule?

That's the thing, I coming from the point of view of a Tau player in which case I'm regularly firing mixed types with fire warriors...

Note, with a barrage template you should be dicing to hit each unit in turn (speed rolling by unique type if need be). This is so that units with different characteristics that can affect the game are diced separately to hit (and save). This was extensively discussed and agreed some years ago and is intended to make characters and other special units run the same risks as other units.

However, where Disrupt (or Lance etc) applies to standard shooting, the defender is allowed to choose how hits are allocated within the "front-to-back" allocation process. So if your do shoot up an Ork Warband with a Firewarrior formation that is upgraded with Broadsides or Piranhas, then the Ork player can allocate the disrupt hits to the Grots leaving 'normal' hits on the Boyz, in order to minimise the potential impact as described above.

Technically you possibly ought to do something like this, though this situation is highly unlikely to occur. There are usually many more Boyz and Nobz etc than Grots in a formation, and the placing of the Grots combined wth the hit allocation process will mean the hits are more likely to be placed on Grots than other units.

BM's are placed after all hits are saved or failed, so it would not matter if non grot units are rolled for first.
If there are no ork units left when BM's are added killed grots count for BM's.

Was just looking at the NetEA Tournament Rule Pack.
This introduces the Expendable special rule. (2.1.22)

"If an expendable unit is hit by a weapon with Disrupt it does not inflict a Blast marker"

I take it that this doesn't apply for EUK tournaments as EUK uses the rules from the GW website