You both ordered drinks or You both ordered a drink or in stated differently:
Both of you ordered a drink or Both of you ordered drinks
should a drink be in its plural form?
Both singular & plural sound correct to my ear. There's a slight difference in meaning, or at least emphasis:
These don't specify how many drinks were ordered - the possibility is left open that one or both of you ordered several drinks.
Here it's pretty strongly implied that there was only one drink per person. (Especially with the latter, it could even be argued that there was a single drink which you shared. My opinion is that such an interpretation would be incorrect, but I don't have anything to support my belief.)
I'm not really disagreeing with Martha, exactly; but I do believe it would be much better in this scenario to make the pluralization of drink agree with that of the subject(s) of the sentence.
"Both" is plural. Therefore I consider this sentence to be ambiguous:
The reason it's ambiguous (to me) is that both is plural, but drink is singular. The above sentence could be interpreted to mean that, between two people, a single drink was ordered (perhaps the two decided to share).
(Martha remarked that "My opinion is that such an interpretation would be incorrect"; and I suppose this is where we are in disagreement. Personally I feel that while such an interpretation would certainly be odd in the context of everyday conversation, from a rigid grammatical standpoint, it would actually be the most correct interpretation.)
The following option is clearer:
Clearly more than one drink was ordered, total. However, I have to concede that there is still some ambiguity in the above sentence since it is not entirely evident whether each individual ordered one drink or multiple drinks.
I consider this last option to be the best of all:
Though it's surprising to many people, "each" is singular. When the subject of your sentence is "each," it is as if you are referring to every individual within the group and making the same statement in each case—something like this:
The above is completely unambiguous, at least when it comes to the number of drinks ordered by the individuals comprised by the word "both".
My preferred phrasing for the situation I think you're describing is "each of you ordered a drink."
But none of the four you listed sound wrong to me.