Question: what is the difference between ionic 3 and ionic 4?
Answer: IMO, the main difference is that Ionic 3 is abandoned (and it was abandoned almost one year from Ionic 4 release), and have a lot of bugs. The last release of Ionic 3 was on 8/Nov/2017 while 4.0 was released on 23/Jan/2019, more then a year without updates.