[Long Haired Git]

I need a picture to understand.
Are you talking about buying say four heater cores, and then removing both end tanks from two of them and one end tank from two of them, and then joining up the tubes in order to make a single radiator? This radiator would be one radiator tall, one radiator wide, but four radiators deep so that air flow goes through all four radiators, right?
First, I wouldn't worry too much about the liquid pressure drop.

For example, imagine having four MCR120QP radiators and make them joined up using 10cm of 3/8" tubing. This will be worse for flow rate, right?

MCP350, Storm waterblock, single MCR120QP, 1m of 3/8" tubing = approx 3.4LPM
Using MCP655 @ 5 instead: approx 5.6 LPM

Now add in another three MCR120QP radiators each with 10cm of 3/8 tubing,
MCP350: 2.5LPM
MCP655: 4.3LPM

So, around 25% reduction in flow rate.

Adding them the way you describe will, IMHO, result in even less degregation of flow rate. Example is having two MCR120QPs vs single MCR220QP. I was going to use my Approximator to show you what I meant, but the MCR120QP has 3/8" barbs and the MCR220QP has 1/2" barbs and my approximator can't yet handle swapping barbs over and thus the figures are not accurate and hence a MCR220QP has BETTER flow rate than a MCR120QP!

Now, two things of import:

1. Super long radiators may suffer from performance reduction due to a lack of turbulence in the tubes as the uniform shape results in laminar flow. Hence, the coolant touching the tubes gets cooled, but as water is a crap conductor, it insulates the rest of the coolant in the tube. This is one reason why dual pass radiators which mix their coolant in the bottom end tank do so well.

2. Single pass radiators such as what you might be describing cause lower overal tube velocity than double pass radiators and this again reduces turbulence and thus performance.

3. Whilst water has awesome specific heat propertes, and hence doesn't increase in temp much whilst dumping energy into it, air does not have this quality. The air temp coming out of the first radiator will be significantly hotter than the air going in. The entire system works on the delta-to-air-temp, and as you're heating the air I would suspect that air-flow-series radiators to be completely awful for their performance.

4. Our fans are weedy, pathetic creatures. One radiator is bad for air flow, but four stacked would be like a brick wall. You'd seriously reduce the overall air flow, further reducing performance.

Overall, best avoided.