how do we 1v1 in bf4?

Well, as far as I know, it is impossible to do pure 1vs1's that would count the score, rented server or not. The thing is, 4 players are needed to start a round (change that was implemented to prevent people from boosting). Basically, you would need to find another 2 people to kickstart the round and then have them stay out of the way. Then, unless you have your own server with password, other people will be able to join, which obviously makes 1vs1 tough.

That aside, in terms of Battlefield 1vs1 isn't really that great as learning method. The only thing you really will get from it is some knowledge on gun/vehicle behavior, something that can be done just as fine on Test Range. In actual Battlefield rounds, threats usually come from different locations at once. Reading the flow of the map is also a massive factor. For training, Team Deathmatch rounds would probably benefit you way more (speaking from experience here).