Just an update for those who might hit a snag in the future:
The SE IV didn't work (was freezing) on a PowerMac 7100 with a Sonnet G3 upgrade card.

However it did work with the 2.1rc2 firmware update. The 2.1rc2 update makes the SE IV SCSI Manager 4.3 compliant, which Sonnet's G3 driver requires. In order to use the 2.1 upgrade you must manually choose the hex address of the slot the card is in; it won't find it on it's own like the 1.65 updater. Holding down the shift key on program launch and typing 0B hex did the trick.

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