Based on the details you have provided, your system is as follows:
- 4 Ohm coaxial speakers rated at 60w RMS (4)
- 4 Ohm subwoofer rated at approximately 400w RMS SVC (1)
Your options include those you have mentioned which are common:
- A single 5 channel amp
- 4 channel amp + monoblock amp
Having a separate amp for the mids/highs and lows allows you to have more flexibility in your system for future upgrades. For example, if you wanted to get an additional subwoofer, having a 5 channel amp with a lot of power in a single channel (800w+ RMS) can be be rare and more expensive. A single 5 channel amp can be cleaner and easier to install.
This is basically what you need to look for in an amplifier if you go with either option:
- RMS Power @ 4 ohms 60 watts x 4 channels (Full range channels)
- RMS Power @ 4 ohms 400 watts x 1 channel (Subwoofer channel)
You'll want to match or exceed the RMS power of your speaker with your amp. The specs above are the bare minimum of what you should look for. This article from Crutchfield can provide you with more information on what to look for in an amplifier. They also suggest that your amp power should be rated at 75% more than your speaker's RMS rating (although I have never heard this number until now, use your discretion).
Do you need a crossover?
The majority of coaxial speakers already contain crossover within them so an external one is not needed. Multi-channel amplifiers will usually have high-pass and low-pass filters built in which serve as a crossover. It all depends on what you want in your system and how much you're willing to spend. Here is another article from Crutchfield with more information on crossovers.
If I split the rear inputs with a cross over will it lower the impedance and burn up the head unit?
With electronic crossovers (like the one you're leaning towards), the load of the speakers will be handled by the amp and the crossover will have no effect on impedance. With passive crossovers, you would probably want to ensure you are getting the impedance you require by testing with a multimeter if you are using something custom or not designed for your speakers. Passive crossovers usually are included with speakers that require them.