Translation:Tomorrow will be April fourth.
I'm from the US and to me "April the fourth" is more natural than "April fourth." Although I avoid saying either because I'm terrible at dates.
I wonder if the error is because of you saying "tomorrow is the fourth" instead of "tomorrow will be the fourth."
Wait, so is tomorrow never going to be tomorrow? Tomorrow won't become tomorrow until tomorrow, and then it won't be tomorrow anymore, because tomorrow will be the next day. Sorry, it's late and I'm tired :P I forgot what I was posting about
It is the use of present tense that Duo doesn't like, I got it wrong with 'is' and then right with 'will be'. Duo was fine with me putting the day before the month.
Separately, "tomorrow is..." is a natural thing to say in English, I don't know if it is good English though :-)
It's よっか (yokka), not よんか. The days of a month have particular pronunciations (especially the 1st through the 10th): https://en.wikibooks.org/wiki/Japanese/Vocabulary/Dates#Days_of_the_month