1) Is my brain still in 'development'?
It is. Your brain constantly grows until you are 25.
According to recent findings, the human brain does not reach full maturity until at least the mid-20s. (See J. Giedd in References.) [...] 1
The rational part of a teen's brain isn't fully developed and won't be until he or she is 25 years old or so. [...] In teen's brains, the connections between the emotional part of the brain and the decision-making center are still developing. [...] 2
2) If so, what negative effect, if any, will having a single beer 3-4 nights out of the week have on my brain development and my health? Will I have to suffer the consequences of these decisions as I continue through adulthood?
I can't cite an exact study right now as I haven't found one but several sites hint that there shouldn't be a problem if alcohol is taken in moderately.
If you choose to drink alcohol, do so only in moderation. For healthy adults, that means up to one drink a day for women of all ages and men older than age 65, and up to two drinks a day for men age 65 and younger. [...] 3
However, I found a study trying to unveil the links between alcohol consumption and its effects and the age in which drinking starts.
First use of alcohol at ages 11–14 greatly heightens the risk of progression to the development of alcohol disorders and therefore is a reasonable target for intervention strategies that seek to delay first use as a means of averting problems later in life. 4
As you age, the probability of developing an alcohol addiction shrinks.
[...] Ten years after their first drink 13.5% (49 of 363) of the respondents ages 11 and 12 and 13.7% (155 of 1,129) of the respondents ages 13 and 14 had progressed to a diagnosis of alcohol abuse, compared to just 2.0% in the reference group (19 and older) [...] 4
Here is an excerpt of a study I didn't have access to. The study, however, seems to represent some relevant data if you can access it.
[...] Discouraging alcohol consumption until neurobiological adulthood is reached is important for minimizing alcohol-related disruptions in brain development and decision-making capacity, and for reducing the negative behavioral consequences associated with underage alcohol use. 5
Finally, I found a PDF concluding a lot of relevant data 6.
Adolescent alcohol use sets up a persistent increase in activation of brain signals that contribute to inflammation. The consequences of this are unclear, but similar changes have been associated with depression in adults, and suggest that early alcohol use may set kids up for mood problems in adulthood. 6
The PDF should give a good insight about the topic. It concludes several other things aswell. Unfortunately, I wasn't able to find an exact study to meet your requirements. But I guess I could say that drinking in moderation is okay, but it's always better to stop it completely 7.