xover question

I propose to wire two 4 ohm fullrange drivers in series. My plan is to
use a coil after the first driver in order to prevent comb filtering and provide a more even frequency response. I believe that I read somewhere that
it doesn't matter if the filter is place before or after the driver in a first order crossover. Does this mean that I will be attenuating the highs in the first driver ahead of the filter as well?

Just beware that by doing that you can make a difficult load for the amp. You are, in effect, making a short circuit in the high end. A small value resistor in series with the capacitor can help.
No, capacitor must be parallel only to one of the drivers and not to both of them. High frequencies on that driver will be attenuated at a rate of 6 dB per octave (first order low-pass filter). There is no short circuit at high frequencies, total impedance will be the same as in the other driver alone.
With two drivers in series, if you put capacitor in parallel with ONE fullrange driver, that driver will have attenuated highs (first order low-pass filter), in order to prevent comb filtering.