Difficult decisions
Should I upgrade to SF5 ?
Yes, of course, but it is a headache process due to the lack of updated major bundles
Do you realy need user registration ?
From our experience almost every project needs login and registration and this logic is not part of Symfony. A great work has been done with fos_user_bundle but it is not supported in SF5.
Have you developed your own login system?
That's great! But are you sure it's safe? Properly done security is a "necessity" and is nothing more valuable than community feedback on finding any issues.
The truth about user registration and login
Only those who have tried to make one know how many scenarios need to be covered and how difficult it is to implement them correctly: the forgotten password, remember me, e-mail confirmation and internationalization.