Those Pitman motors are probably 1/8inch.There are standard motor shaft sizes, There problem is, there are many standards... Metric sizes will probably be either a whole or half millimeter and under 1/8 inch they will probably be in 1/32 inch increments (or 1/64) over 1/8 they will generally go by 1/8ths with a couple 1/6ths thrown in (we seem to like 7/16 and 9/16)
You can get gears/pulleys for a smaller shaft and drill/ream them out. 4.1mm sounds unlikely - micrometer or calliper guage is the way to measure.
Are there shaft adapters for DC motors usually used in toys so that I can use rods that are larger than motor shafts?