Yes, the amp will see different loads depending on the frequency. You'll need a crossover on the midrange too or you'll end up with comb filtering. General rule of thumb is you want cross around 2x the FS of the tweeter. If you end up crossing at ~5khz or higher, then you can probably forgo the crossover on the midbass driver.
You will not be able to drop the impedance with your current equipment. I would ignore the impedance of the tweeter and focus on just the midbass driver. You could get another 4 ohm midrange driver and run them parallel or get a 2 ohm midbass driver. In your case, I'd probably just buy a 2 ohm comp set. Something like these:
I suggest a comp set because the crossover is designed for the drivers being used and the crossover is as important as the drivers. Buying off the shelf crossovers can be a disaster waiting to happen. You have to take into account if a tweeter attenuation circuit is needed and will it be driver side or amp side. There maybe a tweeter protection circuit built into a crossover that comes with a set. The crossover frequencies are important too.