Each occasion of modular causes the focused creature(s) to get the total quantity of counters. In your case, you’ll be able to select one goal artifact creature twice and provides it 4 +1/+1 counters whole, or two creatures for 2 +1/+1 counters every.
Modular means that you can put the useless creature’s counters on a goal creature.
702.43a Modular represents each a static capacity and a triggered capacity. “Modular N” means “This permanent enters the battlefield with N +1/+1 counters on it” and “When this permanent is put into a graveyard from the battlefield, you may put a +1/+1 counter on target artifact creature for each +1/+1 counter on this permanent.”
Putting counters on a creature just isn’t the identical as shifting them. Moving counters solely works as soon as, whereas placing them doesn’t take away the counters from the supply and might thus be executed a number of instances.
122.5. If an impact says to “move” a counter, it means to take away that counter from the thing it’s at the moment on and put it onto a second object. If both of those actions isn’t attainable, it’s not attainable to maneuver a counter, and no counter is faraway from or put onto something. This could happen if the primary and second objects are the identical object; if the primary object doesn’t have the suitable type of counter on it; if the second object can’t have counters put onto it; or if both object is now not within the right zone.
122.8. If a triggered capacity instructs a participant to place one object’s counters on one other object and that capacity’s set off situation or impact checks that the thing with these counters left the battlefield, the participant doesn’t transfer counters from one object to the opposite. Rather, the participant places the identical variety of every type of counter the primary object had onto the second object.
So when the Mouser dies with two +1/+1 counters on it and two cases of modular, each cases of modular set off on dying. You could select two completely different authorized targets, or the identical goal twice, and every cases provides its goal two +1/+1 counters.