Short answer- the 4Ω is based on assumptions that aren't true, but it probably wouldn't be too far off.
The 6A rating of the Peltier device will simply be a maximum allowable current based on device construction, ie any more and you could damage it.
Add to that the fact that a Peltier device can also generate a voltage which may be positive or negative depending on the temperatures of the two sides (see https://en.wikipedia.org/wiki/Thermoelectric_effect). Plus it will also have a small internal resistance.
A 4Ω resistance will provide at most 3A from 12V to a Peltier device at 'neutral' conditions (ie no temperature difference, but note that will change after power is applied).
If it were me, I would do some tests with an ammeter and an assortment of resistors. Keep in mind that 3A at 12V is 36W which needs to be dissipated somewhere, so the resistors will get hot. You'll probably need a handful of 10W or higher resistors.
Because you have 9 peltier devices, another option is to put them in series/parallel. Perhaps start with three strings of three. Then you avoid losses in the dropping resistor.