Microtransactions are indeed coming to Uncharted 4's multiplayer mode, Naughty Dog has confirmed.
“Microtransactions will be available at launch, but we’ll have no gameplay items that will be gated through microtransactions,”
- Robert Cogburn, Lead Multiplayer Designer at Naughty Dog
But what wasn't immediately apparent was how to acquire those items for use. The good news is that one can use in-game currency for all the different weapons, special items and upgrades. This does bring into question whether those items will have a significant effect on gameplay and if that means this could become a pay-to-win scenario. Apparently the multiplayer beta that's coming in December will be a testing platform to ensure that the right balance of usefulness is found.