If you have some kind of coaxial speakers that can fit the 6x9 holes with an adapter, you can always give them a try. In fact, why don't you wire the 4 inch speakers to the stereo? Audiophiles usually run them attenuated, so the brand/size/amplifier does not make a big effect on the result.
For some time I am have been using cheap coaxials as my rear speakers. They were time delayed (20ms or so), high passed at 250Hz, and attenuated to extent where they did not draw the sound stage to the rear. If I was running the same setup, I might have added an inline inductor to each one of them to create a bandpass setup from around 250 to 2500Hz. The added ambiance effect wasn't bad, but since I have switched to running 2-way active front I have lost the ability to send signal to the rear speakers. I think I might try running rear speakers again once I have an advanced sound processor like JBL MS-8 or BitOne/BitTen. One common recommendation is to have the rear speakers made by the same company from the same kind material, etc. I don't know if matching brands for front/rear is that important.
On the other hand, if you want the rear fill primarily for bass, I would say get a decent 6x9 speaker, bandpass it 60-200Hz or so. The fact that you have a stereo powering it could be a problem though. I am personally not a big fan of rear mounted midbasses. (note how my rear fill was high passed from 250Hz). Besides the sound staging issues, I think this could make the midbass frequency response worse or color it in some way. Bass frequencies are a lot more sensitive to phase coherency issues than higher frequencies. With 5 speakers playing midbass frequencies (2 front, 2 rear, and subwoofer) you now need to worry about creating a phase coherent setup with five speakers! The complexity seems not to be worth it. There exist several 6.5 round and 6x9 oval speakers that could provide adequate bass if installed in front with good sound proofing and power running to them.