I am a student who only attends school part-time, and I am on my 4th year right now. 4th year undergrads are typically considered seniors, but I feel like it is a little weird to call myself a senior when I only have 107 quarter credits (I need 180 to graduate). As you could imagine, I sometimes get asked what year I am on. I usually say that I am on my 4th year but am only a junior by credits, but that wording feels weird to me. I sometimes don’t know what I should say when people ask me what year I am on.
I would go based on credits and I say I’m a junior. You don’t have to tell anyone how long you’ve been in school.
I think you would be considered to be a junior. It is fine to say you are going to college part-time and are a junior. Part time students typically take longer than four years to graduate.
@happy1 Students in general typically take longer than four years to graduate. Believe it or not, the average is six years.
College was the best 7 years of my life 
@TheAverageJoe71 I understand. I was just referring to the OP’s specific situation which was delineated in his/her post.
Divide your credit units by the number per year (180 / 4 = 45 in your case) and round down:
0 = frosh
1 = soph
2 = junior
3 or higher = senior
With 107 quarter units, this makes you a junior (107 / 45 = 2.377… rounds down to 2).
At my old university people often said “I’m a first, second, third, fourth year” as it takes many 5-6+ years to graduate from there.