Most cartridges that have an extremely low source impedance, such as the 3-4 ohms you mentioned, are quite low in output (fewer coil turns mean low impedance and low output), and therefore also need more gain. The transformers that "match" to such cartridges typically have higher turn ratios and gain. But, if excessive gain is not a big issue, you can use such transformers with cartridges with a higher source impedance and the result should be fine.
I believe Art Dudley, in a series of articles in Stereophile about SUTs, said that, not withstanding supposed compatibility criteria, he has not heard problems with mismatches.
I have personally never experienced compatibility issues except for not having enough gain. Most SUT with enough gain have worked well, and personal favorites seem to remain favorites with a wide variety of cartridges.
I believe Art Dudley, in a series of articles in Stereophile about SUTs, said that, not withstanding supposed compatibility criteria, he has not heard problems with mismatches.
I have personally never experienced compatibility issues except for not having enough gain. Most SUT with enough gain have worked well, and personal favorites seem to remain favorites with a wide variety of cartridges.