4-Track Recordings

There is only one way to do this my friend and that is using a 4 input device.
You have already figured out that the capstan fluctuations/drive motor have minor differences on each pass.
My suggestion is to either send them out to be transferred. However to be quite honest, for the money it will cost to transfer 200 hours (depending on IPS that's more than 200 tapes) some company's can charge up to 30 bucks a tape:
http://www.audiomover.com/transfer-4track-to-digital.html
You would be better off buying a 4 input device.
 
If you don't want to buy a device but want it done right (with good pre amps) just send it away.
 
There is one other way that people sometimes over look.
Using an older style Digital recorder that has 4 inputs. I had a Tascam 2488 that had 4-8 inputs on it a while back before I switched over to a newer unit (Focusrite Scarlett 18I20). I would use the Tascam to transfer and then burn it to disc.
 
Also remember if you send it out leave detailed Pitch information. I always ask my customers for IPS info.
Some companies just set it at 1 7/8 and call it a day. you need to be clear you want 3 3/4 and what pitch (what you set the Vari-speed knob at).
Back in those day we cranked that thing up to gain maximum speed/better fidelity. 
 

I did a similar transfer when I went from a Fostex VF160 8 track recorder to Sonar.  
I transferred two tracks at a time as I had a two track recording interface unit. 
I did have to do some (very little) track alignment but that was all.
 
It was time consuming but didn't really cost me anything but my time. I only had about 10 songs to transfer.  However with a large amount of recordings it would have been different and I would have had to consider whether I wanted or needed to transfer that large amount. 
 
 

I recorded by old 4 track cassettes, using my old, but trusty Walkman Pro cassette player into Sound Forge. I split the left and right of both sides into separate tracks. Of  course two of the tracks needed to be reversed. I cleaned up the hiss, and imported the 4 tracks into Sonar, and aligned them up, and re-mixed. It's very tedious, but it worked. 

Thanks again to Charlie for the article. I don't think that I would have the time for all the fancy editing and aligning required for using my current equipment. Converting 200 hours worth of 4-track would be quite an investment in time, plus the live stuff would just be impossible. I should have thought about this when I upgraded my system last year and bought a 4 input interface. Never mind - Christmas is coming!

I wish you lived closer, for 4 track transfer I charge 10 USA dollars a tape (60 min in half=30 on 1 7/8 ISP).
That includes RAW track conversion. for a little more I can do noise reduction and restoration.
I do everything short of baking tapes which I did years ago but stopped.
Most places charge up to 30 so you could see how investing in a device is worth it.
 
Just a suggestion but this is what you want-
Focusrite Scarlett 6i6
https://us.focusrite.com/usb-audio-interfaces/scarlett-6i6
 
They can be bought used for under 200. They have 2 pre amps, 4 line inputs and 2 outs
this equals= 6 total inputs. They also have 2 headphone inputs with separate volume controls.

Deleted, Duplicate

Thanks to all who responded here. I've now got plenty to think about although a new Focusrite is on the cards.