I questioned, and continued to question, why you assume that Device Rating describes a specific processing level - I don't find that assumption justified.
Specifically, I would suggest that Device Rating actually describes the relationship between the resources the system has available and the sort of demands that are placed upon it - and since a commlink has much less demand placed upon it, it can more directly dedicated towards the things represented by Device Rating, Data Processing, and Firewall, while a deck has to dedicate it's resources towards Device Rating, Data Processing, Firewall, Attack, Sleaze, and its Program Rating. An RCC must dedicate those resources towards its Device Rating, Data Processing, Firewall, Sharing, Program Rating, and various other functions.
Thus, that DR4 Deck actually does have more processing power than a DR 6 commlink; the relationship is similar to that of a commlink versus a proper computer.