Flies with an extra fourth chromosome are fully viable and look like flies with two fourth chromosomes. Flies with three fourth chromosome arise by meiotic nondisjunction—a somewhat common event. Consequently, it is not unusual to find stocks with trisomic flies, and, if trisomy is selectively advantageous, a high proportion of the flies in a stock may be trisomic. 

We advise you to watch for signs that stocks you use in your crosses have trisomic flies. Unexpected progeny ratios will likely be your biggest hint.

We explore the issue of fourth chromosome trisomy in stocks in the short paper The underappreciated, underrecognized problem of fourth chromosome trisomy in Drosophila melanogaster stocks and a simple, general method for building diplo-4 stocks from triplo-4 stocks. It also provides an easy way to build disomic stocks from trisomic stocks if you find you need to clean up a stock.