The Hebrew word for four is arba ארבע (masculine arba'ah ארבעה). The root of the word is רבע, which leads to such words as reva רבע - quarter, rvi'i רביעי - fourth, and m'ruba מרובע - square. This is fairly straightforward.
However, as we progress in our discussion of numbers, many words for numbers will appear to be very similar to other Hebrew words. In some cases it is just a coincidence, and we will be able to prove it. In other cases the connection is much more strong. There are those that claim that the words for numbers derive from these other roots. In general, I am not inclined to accept these theories - although I will present them here for you to view for yourselves. If anything, I think it is more likely that the words for numbers came first. Hebrew is an ancient language, and it seems very unlikely that Hebrew would lack such basic words as the numbers from one to ten, and need to borrow them from other words.
Arba is a good example of this discussion. There is another meaning for the root רבע - to lie down, mostly for animals and as Klein writes "in Heb. it usually refers to copulation, mostly unnatural." Its cognate is the root ravatz רבץ (the ayin and tzadi switch in Hebrew) - also meaning to lie down or to crouch, generally for animals. Klein makes no connection between "to lie down" and "four", but others do. Steinberg, in the Milon HaTanach, writes that ravatz means to lie on hands and feet - on "all fours".
Strong (or is it the BDB?) writes that רבע is:
A primitive root (rather identical with raba' through the idea of sprawling "at all fours" (or possibly the reverse is the order of deriv.).
I suppose the best slang equivalent in English would be "knock". To "knock out" can mean "to hit" or "to exert or exhaust (oneself or another) to the utmost" and "to produce in abundance".
As I mentioned earlier, I think it is much more likely that "sprawling on all fours" derives from the number than the other way around.
Another meaning of רבץ / רבע is "to irrigate, to sprinkle". Klein does not appear to connect this root with either previous meaning. Jastrow does connect "to lie with" and "to inundate for the sake of improving the soil" - both have the meaning "to cover". From the meaning "to irrigate", Jastrow derives the term to teach Torah - מרביץ תורה - marbitz torah. This is the origin of the name of the prestigious Biblical studies journal, Tarbiz.
In colloquial Hebrew, marbitz also has the meaning "to hit, to spank", I assume because it may cause someone to lie down on the ground. In even more recent slang, marbitz means "to really do or make something". Rosenthal includes such entries as:
הרבצנו מיץ תפוזים - we drank a lot of orange juice
הרביץ הופעה - dressed very impressively
להרביץ קפה - to make coffee
נרביץ תמונה - let's take a picture