About PS3>PS4 Upgrade

I want to understand this process, I have the PS3 version of the game, if I upgrade to the PS4 version I'll be unable to use the PS3 right? Then if I want my brother to play on the PS3 and me on the PS4 what should I do? The PSN account is linked to the game? Or if I uninstall it and re-install it he will be asked to link his SE account to his PSN account?

Probably is too confusing to undesrtand, I'll short:
-I have both version PS3 and PS4
-I want to use both, so my brother use PS3 and me PS4
-If i start the PS4 whit my PSN/SE account the PS3 version will no longer be playlable?
-If my borther try to log in on PS3 he will be asked to link his SE account to his PSN account or FFXIV is account lock?

I know is confusing, feel free to ask, I just want to understand what I need to do.

#If this is not the tread, please relocate.

The game can't be used if ur upgrading from ps3 to ps4. The ps3 version becomes void. But if u purchase both. U can use both. But once uve upgraded u'll need a new version for the ps3.

If you upgrade from PS3 to PS4, the only thing that happens is that you are unable to play the PS3 version using the account that was upgraded. You are still able to access the downloaded version of FFXIV on the PS3 using another account/user on the PS3, provided they have a PSN Account linked with an SE account and it is currently active.

The only thing that would need doing is for another PS3 version of the game to be purchased by your brother (digital or disc, it doesn't matter as both provide a download voucher code). Use the code on his PSN Account, and then start the FFXIV client. It'll ask if the account is linked or not and if you wish to make an SE Account, provided it is not linked, which will then be linked to the PSN Account in-use.

You do not need to download the client using the registered code on his PSN account. However, you still need to register it as it is proof of purchase and that's what enables you to play at all with said account.

You upgrade your PS3 version (linked to your PSN account) to PS4. Then you can play your own account on the PS4.
Your brother buys a new PS3 version (from his PSN account, or linked to his PSN account) and links that to his own SE account (or sets up a new SE account for it).

Then you'll each be able to reach your respective FFXIV accounts from your individual PSN accounts.

(Your brother wouldn't need to re-download the software if he's using the same PS3 you'd already loaded it on. It just needs to show the purchase on his PSN account, either buy purchasing through PSN or by registering the code from a boxed version to the PSN.)