Very few people (even some 'experts') have the correct knowledge to tell the difference between a Ford C4 and a C10 Automatic Transmission. Alot of listings contain incorrect descriptions about the type of transmission being offered, mostly because the seller does not know or has been misinformed about thier transmission. Many people regard the C4 as a 'Tough Box', but it is not as durable as a C10. Half the reason the C4 has that reputation is through misidentification- the C10 being mistaken as being a C4. Ford originally fitted the C4 to 6 cylinder engines and small cubic inch V8's generally under 300 C.I. The superior C10 was used on all lower performance V8 engines after aproximately 1970. Over the years, Ford produced many 'C' code transmissions, including: C4, C5, C6, C9, C10.

When a C4 and a C10 are placed side by side, the differences are extremely obvious as is evident in the pictures. 

Above is a C10, note the smooth transition between the transmission housing and the bell housing.

...and this is a C4, as can be seen by the sharp step up between the transmission and bell housing.

The following information is sourced from a Ford factory repair manual.

The Similarities:

• Both have 11 bolt sumps.

The Differences:

• C10 dipstick/filler tube enters the side of the transmission sump.
• C4 dipstick/filler tube enters the topside of the transmission housing.
• C10 has a smooth transition between the Bell Housing and Transmision Housing. (see pics)
• C4 has a step up between the Bell Housing and Transmission Housing. (see pics)
• C10 has a breather valve.
• C4 has a breather tube.
• C10 has a large input shaft with a smaller 'snout' protruding from it. (see pics)
• C4 has a smaller input shaft with no snout. (see pics)

Below is the C10 shaft...

...and this one is the C4