ok sat down and done the maths:
4e rules, opposed test. day to day scanners (R1/R2) will tend to glitch/fail test just as much as a cheap SiN would, causing no problems to runner, though largely luck based due to low dicepools
5e rules, SiN as threshold for scanner is fine means better scanners will still struggle against a threshold 4 test (requiring an R6 scanner to burn an R4 scanner on average) but low rating SiNs will be burned through very quickly in the nice part of town.
Earlier ruling rescinded, 5e rules for SiN in use from now on:
R2 scanner gets 2 hits:
4d6.hits(5)=2 against threshold of 4.
Taejix, you're free to catch the train!
IC thread updated