Rinx,
Now you are getting to a very tricky part. First keep in mind that it only takes 1 watt to push your speakers to their rated sensitivity. Typically sensitivity rating are in the form 90db at 1 watt at 1 meter.
Two speakers in Parallel receive the same voltage, but the total current is divided between them. Two speakers in Series divide the total voltage, but they both get the same amount of current.
When you turn the Volume Control, you are not applying watts to the speaker, you are applying signal or voltage, and the resulting watts are what ever they are based on the load.
If you apply 16 volts signal to 8 ohms, the resulting power is
P = (E^2)/R
P = (16^2)/8
P = 256/8 = 32 watts
16v to 4 ohms
P = 256/4 = 64 watts
16v to 16 ohms
P = 256/16 = 16 watts
Notice that while each example above consumes a different amount of power, they all have the same signal voltage applied to them.
So power and perceived sound are not linear. One nudge upward of the volume control does not result in one nudge upward of the power.
When you turn your Volume Control up one very very small amount, just enough to hear a change in the sound level, that is about 3 db. Three DB represents a doubling of applied power.
So, in the above case, the perceived volume (what you actually hear) from an 8 ohm speaker relative to a 4 ohm speaker is one very tiny nudge of the Volume Control. A 16 ohm speaker relative to a 4 ohm speaker is two tiny nudges upward of the volume control. In either case, you are always going to be well within the normal turn of the Volume Control; that is, still in the normal listening range.
All that said, I still say your amp shouldn't have a problem with 4 ohms, but obviously, I can't actually promise that.
Another question, is this speaker design a known design that you are copying or basing your speakers on, or is this an original design created by you?
If it is a known design, and there are lots of them out there, then you should be able to get information on those designs and get some sense of the impedance response curve. This does sound like a fairly standard design.
Exactly what Dayton speakers are you looking at, and have you actually purchased them yet? It is possible that you might be able to find the equivalent Dayton in a 16 ohm version and that would solve your problems.
Still, in the end, the world is full of 4 ohm and 6 ohms speakers, I can't imagine that any good amp would have a problem with that.
One thing you could do, is when your cabinets are rough ready, temporarily install your woofers without the crossover, and listen to them in both parallel and series at modest volumes (25% to 30% turn of the Volume Control), and see if you hear a real difference in them.
It's a tough decision to make, but it is one that all speaker designers face.
Sorry, the was probably a lot of talking that adds up to not much help.
Steve/bluewizard