Can you use a 8 ohm crossover with 4 ohm speaker?

That 18 inch driver I showed you guys earlier[The RCF in the twin slot vented Martin Audio enclosure] sounds pretty nice but mid frequencies are coming through and making it sound crap.Box is tuned to 30hz and says that on the back under the Martin Audio writing on the back panel.

So I'm gonna use a passive crossover[passive low pass filter] cut off at no more than 120hz. But the problem is the driver is 4 ohm, and the passive low pass filter I wish to use is rated 8 ohms.

So will an 8 ohm passive low pass filter cope with a 4 ohm bass speaker without sounding horribly wrong and will it turn the 4 ohms into 8 ohm load???

Gonna be powering it with one channel of a QSC RMX2450 [750w rms into 4 ohm] if the passive LPF doesn't change the impedance.
If the passive LPF changes the 4 ohm driver to 8 ohm of what the filter is rated at then I'll use the sub in bridge mode giving it 1500w rms into 8 ohms.

RCF driver is rated 700w rms and is gonna be my sub for disco use.
 
It's not that the filter will not be able to 'take' the 'reduced load' or that it will change the impedance (it won't as such), merely that the intended -3dB frequency will double.

As you are giving the driver it's own amp channel you will get massively better result with a simple active filter in front of the amp. Passive crossovers and subs are just a recipe for old pants.