Translation:What day of the week is April fourth?
Thanks! Also here's a great page for covering counters with days and months etc. http://www.guidetojapanese.org/learn/grammar/numbers#Counting_and_Counters
I wish Duolingo would integrate learning all the different rules for counters in the course. Currently you have to completely rely on posts like this in the forums or external resources.
4, because of its strong association with death, is considered very unlucky. To the point that house numbers, apartment numbers, even floors of buildings just skip over 4 - they don't exist! Makes me wonder who creates these lessons?? Nihonjin wouldn't include this sentence. They'd avoid using or mentioning 4 (Except where it's absolutely necessary) like the plague!
Four and Nine, because four sounds like death 死 (し) and nine can sound like suffering 苦 (く). Hospitals in Japan tend to avoid having rooms with these numbers.
This factoid is courtesy of Easy Japanese, the radio podcasts that teach beginners how to comprehend the japanese they hear.