A mistake commonly made is that cows have four stomachs. That is not entirely true. In fact, cows have four compartments of just one stomach called the rumen, reticulum, omasum and abomasum. Here are the ways that each part of the stomach functions according to USDA:
    
Rumen
The rumen is the first stop, and the biggest part, of the cow’s stomach. This area breaks down the plant fibers the cow ingests. The cows will also heave some of the material back up to re-chew it. According to the USDA, feed can spend 15-48 hours in this compartment before moving on to the next.

Reticulum
Resembling a honeycomb, the reticulum is used for catching foreign objects that don’t belong such as plastic packaging, etc. It can also further soften food that has passed through the rumen.

Omasum
The third of the four chambers is the omasum. Resembling the pages of a book, the omasum is used primarily for absorbing and filtering the fluids the cows consume.  

Abomasum
Much like the human stomach, the abomasum is very acidic and has many of the same functions. The acidity helps enzymes break down proteins into amino acids.

Source: USDA