...the DGL-4500 doesnt run Linux (seems to be GNU-based), and the processor isnt MIPS (custom Ubicom IC), the issue/point is moot (for now) for the 4500, with or without 1.22's HNAP fix (unless of course, they modify the malware code specifically for the 4500s'/etc scenario).
Regardless, running 1.22 mitigates you from any current or future HNAP-based attack (using that particular HNAP exploit at least).