Strangely the impedance was omitted from the specs lists of the DX2 that I looked at.
Impedance varies in use and I wonder if the 4 ohm value you've seen is a worst-case figure (probably is).
The only time an 8+ ohm requirement should be strictly adhered to, is on amps where speaker pairs can be switched in parallel - probably not the case for you.
Having said that, in the (rare) case that a user guide specifically requires a minimum of 8 ohms, you wouldn't want the speakers to be dropping lower than 4 ohms.
See if you can research / confirm whether your 4 ohm figure is the minimum that the speaker drops to.