Significance of extra $4?!

[Nightsky]

I'm just curious, wonder if any of the Seniors here can shed light on this practice.

 

In addition to my regular work, sometimes I do freelance consulting to individuals/families.

 

I have this particular family, whom in addition to my regular billings, always adds an extra $4 in the payroll. Regardless of final billed amount, this family, or rather the wife usually insists on paying in cash and without fail every pay packet will add an additional x2 purple $2 notes.

 

So is it a good omen (even number machiam angbao) or is a bad omen (trying to curse me "死"?! )