Which charge would Robert Milikan not have observed in his oil drop experiment

Which charge would Robert Milikan not have observed in his oil drop experiment?
1. 4.80x10-19 C
2. 1.60x10-19 C
3. 3.20x10-19 C
4. 4.32x10-19 C
5. 1.28x10-18 C

Also would like to know why the others WOULD be observed if you could.

Any help appreciated!

Not observed:

4. 4.32x10-19 C

All the others are integer multiples of the smallest value, 1.60x10-19 C

In other words, #2 has one unit of charge, #3 has two, #1 has three and #5 has eight. #4 is 2.7