If the driver of your audio interfaces indeed support all 4 midi connections simultaneously, then yes, you have 4*16 midi channels available.
No hold on, that’s not what I meant :p.
A single 5-pin DIN will only ever carry 16 channels max. I’m just saying that using 4 identical audio interfaces on the same PC doesn’t necessarily mean that all midi IO on every card is available in the driver.
You see a similar thing happening with the MR816x (if you ever visit that forum), where using multiple mr816s, certain features like direct monitoring don’t just add up with the amount of interfaces you use.
The easiest way to find out if it works is by simply trying it. Hook up all 4, open up Cubase and see if you have all 4 MIDI IO’s available. If yes, happy days. If not, tough luck, I doubt you can find a way to make it work.
Note that this is completely independend of any other MIDI IO hardware you use. If you get a USB (or paralel port, good luck with that one :p) MIDI interface then that should work fine together with any other MIDI IO on for instance your audio interfaces.
I have a feeling I made it more complicated instead of clearing stuff up xD
It entirely depends on how that MOTU works in ‘standalone’. I used to have an E-MU 2x2 midi interface that did absolutely nothing at all unless I used software like Cubase to assign the inputs and outputs. As soon as I’d close Cubase, all midi inputs on the E-MU would just be dead ends.
Now my MIDEX8 does work standalone, but not by passing input 1 to output 1, 2 to 2 etc.
It has 2 modes, it either sends what enters in ‘MIDI IN 1’ to all 8 outputs, or what enters in ‘MIDI IN 8’ to all 8 outputs.
In short: it really depends on how the MOTU was programmed to act without the serial connection. I hope it doesn’t also get its power from the serial port, or you can be very sure it won’t work standalone 
You being an electrician I’m sure you could solve that though.