I have both a SBSS 4 and 5. I will give the 4 to my son as a Christmas present. Meanwhile I have plenty of time to compare the two.
4 has a slightly brighter display ( I don’t know why but it is noticeable) and a slightly better battery life. I compared them using a 1:1 setup in order to be fair. Walkie Talkie off (most useless app I’m my opinion), and sound level meter off.
The difference in battery life is almost identical but 4 gets a few hours more if I keep the AOD using the black background info graphic face.
Speed is identical and the only difference between the two really boils down to AOD and compass. For me the cost is no issue and I like the AOD a lot.
At night, I use the watch for sleep monitoring so I switch the theater mode on and the AOD goes off as a result. At that setting model 5 becomes practically identical to 4.
The bottom line is: if AOD and compass is a desirable feature and the price is not much of a concern, then the upgrade is justifiable. But if you are happy with your 4 and you want to justify the cost of this upgrade, stay with your 4.