What is the connection between ULN2004 and ULN2804 drivers?
Here's the pdf for the ULN2004 chip

It looks like the same application and same specs, set of darlington arrays. ULN2004 has 7 circuits as oppose to ULN2804 which has 8. Both carry 6-15V (CMOS, PMOS compatible inputs), Output currents of 500 mA per driver, and Output Voltage of 50V

It might've been a careless mistake derived from laziness since the chips have the same type of circuitries.

Because these chips are practically the same, you can utilize 2804 the same way as 2004. Only in this circuit, you'd have in8, out8 pins setup the same as in7 and out7