Er, I hate being a rules-lawyer, but I think (and please correct me if I'm wrong) the errata for SR4 says something different.
FAQ | Shadowrun 4
Specifically, the following section:
Why buy Hacking when you can default to Logic?
For tests in the Matrix, a program or Matrix attribute is substituted for your regular attributes (
Attributes in the Matrix, p.226, SR4A); when defaulting on Hacking, Computer, or most other skills in the Matrix, you default to (program rating – 1),
not the normal linked attribute.
Grandma Moses has Computer 0 and Edit 6, and wants to touch up an old hologram of her late husband and the slitch he was banging. This is normally a Computer + Edit (2) Test, but since Grandma doesn’t have Computer her Skill defaults to Edit – 1, so she rolls 5 dice on the test.