One consequence of having bigger, more open environments is that you have a lot more freedom in Uncharted 4's combat encounters, and the best, most reliable way to get through them is by being as stealthy as possible. The opposition numbers are massive in comparison to the previous games, and if you stick to straight cover-shooting, you'll likely meet a swift and painful end. Luckily, goons are no longer instantly alerted when you enter a combat environment, and you can often turn this to your advantage.
You can hide in tall grass to sneak through densely-guarded areas, and if you get close enough to an enemy, you can take them out without anyone noticing. You can also climb up to higher vantage points (or use your grappling hook to swing) and leap down to take guards out with a single punch, as well as climbing below them to fling them off whatever ledge they're standing on.
Pay attention to the icons above your enemies so you can stay unseen. Yellow means they're suspicious, red means they've spotted you. If you get seen and bullets start flying, feel free to go loud against the reduced numbers. But if you're really good, you can sneak off to another corner of the map and wait until the guards go back on their patrols, allowing you to continue quietly disposing of them one by one. They work on line of sight and last known location, so exploit that and disappear. You have a lot of tools at your disposal to ghost entire stages, so use them wisely.