As a follow up question...
The onstage mics are picking up from any acoustic instruments as well any amps on stage? From what I’ve read here it’s best to avoid picking up the monitors.
Omnis on stage pickup whatever sound is most dominant there. If there is a PA in use, typically on stage amps will be louder than acoustic instruments sent through the PA and monitors unless the mic is near and on-axis to a monitor. If no PA, then the stage amps will typically be dialed back so as to be in balance with the acoustic instruments, or the acoustic instruments will be amplified through the stage amps. Monitor's might be okay for reinforcement of the recording or not.. they are bit of a crap shoot as to the sound quality of the monitor mix through them.
I’m thinking of doing 4 omnis. 3 split facing forwards and one in line with the middle one facing towards the crowd. Is there any reason to consider switching out one of the center mics (either forward or rearward facing) with a card or hyper?
Yes. Try it a few ways to see what you find, but I like substituting more directional microphones in the middle facing forward/rearward.
Front/back omnis can work but need sufficient spacing between them. I used to run four omnis in a sort of spaced diamond configuration, FOB or at the stage-lip. I did have the omnis (the very omnidirectional DPA 4060s) mounted in 1.5" diameter spheres to make them more directional (the sphere attachments make them more directional to a lower frequency than the larger 4006). I'd typically space the L/R pair about a meter, and the front/back pair anywhere from about half a meter to a meter. Made some great recordings that way and ran that for years.
However, I found that increased directivity in the front/back axis is helpful, that level-discrimination across that axis was more important than the L/R axis (where spacing alone is sufficient). The less stage-sound there is in the rear/audience-facing microphone, the more useful it becomes, so it's helpful to use a cardioid or supercardioid facing the audience in order to exclude the stage sound from that channel as much as possible. Rearward-facing is the first place I'd put a directional microphone. An omni can work well if boundary-mounted to the front of the stage under the lip so that it is facing out into the audience and occluded from the stage-sound by the stage-lip itself.
As for the center forward-facing position, it can be advantageous using 3 identical omnis across the stage in a spaced L/C/R arrangement. That's likely to achieve a nicely balanced timbre and sound across the front sound-stage. But it also can be useful to exclude the room and audience sound from the center microphone as much as possible. So this depends more on the specifics of the situation than the rear-facing microphone. In a great sounding room without a super loud audience 3 omnis across the front might be best. In a more reverberant room or with more audience level a directional center microphone may be better.
My general take away is this- Spacing is generally more useful along the L/R axis. In the front/back axis it is more useful to have increased directionality and limited spacing. It helps deal with the front/rear level difference by reducing leakage of the front sound into the rear channel, limits time-smear of transients leaking into the rear channel, and reduces rear sound pickup in the front center.