You are correct. If you DEFINE OSC 4 for 4Mhz, but actually use 40Mhz, and your serial commands are setup for 9600 baud, you'll actually get 96,000 baud. But running 9600 baud @ 4Mhz, the bit timing is fairly tight and if you're running any kind of interrupts, that'll mess it up.
Better to use DEBUG serial commands @ 4Mhz since they take up less code space.
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