By American, you are probably meaning Caffè Americano, which literally means American coffee. This is actually more of an Italian coffee since this is Italian itself. It is brewed by:

prepared by brewing espresso over hot water

Quote also from above link. So espresso is also in American coffee. Caffeine will vary as the amount of shots used will vary per person.

Espresso is coffee that is brewed by and is:

is coffee brewed by forcing a small amount of nearly boiling water under pressure through finely ground coffee beans.

It has one of the most highest amount of caffeine of any other coffee drink per unit volume, so you would expect American and any type of coffee involving espresso will lots of caffeine. Well that depends on how much coffee you are drinking as espresso is served in smaller portions.

There's no such thing as Espresso Lungo, it's actually just called Lungo. Another Italian coffee, this coffee is and is brewed by:

Lungo is Italian for 'long', and refers to the coffee beverage made by using an espresso machine to make an Italian style coffee - short black with much more water, resulting in a stretched coffee, a lungo.

Now don't be fooled by this as to make this coffee, you need an espresso machine to make this creation. You just need more-than-normal amount of water and short black (single or double dose or shot). This coffee will have less caffeine than the above coffee types.

Ristretto is more of the opposite of Lungo. It uses regular ground coffee with less-than-normal amount of water:

Ristretto is traditionally a short shot of espresso coffee made with the normal amount of ground coffee but extracted with about half the amount of water... The opposite of a ristretto is a lungo, which is typically double the shot volume.

It is also Italian and literally the opposite of a Lungo. It also has less caffeine than American or espresso since it only uses a short shot of espresso and regular ground coffee, similar to Lungo.

So, American and espresso have the highest caffeine content out of the four, and Lungo and Ristrtto have less caffeine than the above. I would say that espresso has the highest of all in fact but American still looks pretty high in caffeine too.

By American, you are probably meaning Caffè Americano, which literally means American coffee. This is actually more of an Italian coffee since this is Italian itself. It is brewed by:

prepared by brewing espresso over hot water

Quote also from above link. So espresso is also in American coffee. Caffeine will vary as the amount of shots used will vary per person.

Espresso is coffee that is brewed by and is:

is coffee brewed by forcing a small amount of nearly boiling water under pressure through finely ground coffee beans.

It has one of the most highest amount of caffeine of any other coffee drink per unit volume, so you would expect American and any type of coffee involving espresso will lots of caffeine. Well that depends on how much coffee you are drinking as espresso is served in smaller portions.

There's no such thing as Espresso Lungo, it's actually just called Lungo. Another Italian coffee, this coffee is and is brewed by:

Lungo is Italian for 'long', and refers to the coffee beverage made by using an espresso machine to make an Italian style coffee - short black with much more water, resulting in a stretched coffee, a lungo.

Now don't be fooled by this as to make this coffee, you need an espresso machine to make this creation. You just need more-than-normal amount of water and short black (single or double dose or shot). This coffee will have less caffeine than the above coffee types.

Ristretto is more of the opposite of Lungo. It uses regular ground coffee with less-than-normal amount of water:

Ristretto is traditionally a short shot of espresso coffee made with the normal amount of ground coffee but extracted with about half the amount of water... The opposite of a ristretto is a lungo, which is typically double the shot volume.

It is also Italian and literally the opposite of a Lungo. It also has less caffeine than American or espresso since it only uses a short shot of espresso and regular ground coffee, similar to Lungo.

So, American and espresso have the highest caffeine content out of the four, and Lungo and Ristrtto have less caffeine than the above. I would say that espresso has the highest of all in fact but American still looks pretty high in caffeine too.