1888 50 Cents Obv. 4

The biggest difference is the wear patterns evident on the face. The 4th obv. was meant to show a more 'mature' portrait of Victoria. What this means is that it shows the pudginess in her cheeks, and the doubling of her chin is very pronounced. Another deciding factor is the upper lip- if it curls, then you are looking at the 4th obv.; if not, then it is obv. 3...

These are the features that I tend to use when identifying portrait types of Victorian halfs in a variety of grades.

Taking your pic and blowing it up a bit, to get a good look at the facial features I am inclined to believe this is the obverse 3.
The chin and cheeks are the give away , the chin is a little more pronounced and the cheeks are alot less jowely than the obverse 4.
In an obverse 4 coin the portrait would resemble someone sucking on a large candy.