Sneaking

Sneaking is an action possible in Fallout 4 that allows the player to avoid detection from any entity. Unlike previous Fallout games where proficiency of sneaking was handled by the Sneak skill, there is not a proper Sneak skill in Fallout 4. However, characters can still sneak, but effective sneaking is handled through perks and other variables to improve its effectiveness.

Sneak states
The sneaking state is displayed while crouching, and alerts the player to detection and possible detection.
 * [HIDDEN] means you're undetected. In this state, attacking a target guarantees the sneak attack critical bonus, pickpocketing is available, and it is safe to steal, lockpick, or otherwise access "owned" property.
 * [DETECTED] means you've been detected by one or more friendly targets. Pickpocketing is enabled, but almost always fails. Attempting to steal, lockpick, or otherwise access "owned" property will fail and may result in friendlies turning hostile or confiscating stolen goods. Sometimes an unfriendly target will see you and this will be shown instead of caution because they haven't identified you as hostile or a target.
 * [CAUTION] means one or more unfriendly targets are suspicious of your presence and are searching for you. After a time the enemy will dismiss the disruption and the state will return to [HIDDEN] . Pickpocketing is disabled for the enemy searching for the player. Attacking in this state will not cause a sneak-critical hit.
 * [DANGER] means your presence is made known to hostile NPCs, but they do not know your current location. They will actively seek you at or around your last known location. If [DANGER] is flashing, it means that the enemy can currently see you and will engage you. The state can be returned to [HIDDEN] by moving away and staying out of sight for a long period of time. Note: As soon as the conditions are satisfied for any state it replaces the one above it on the HUD even though the player may still be in the [HIDDEN] state to several targets around them. Essentially, it reports the worst detection state towards the Player of all potential targets. Once all targets that satisfy the most-elevated state are killed (as in you kill everyone actively pursuing you in [DANGER] ), the state will revert to the highest remaining state among the remaining targets.

Sneak attacking
Like in Fallout 3 and Fallout: New Vegas before it, a sneak attack critical occurs when attacking while [HIDDEN]. Successfully hitting the target will result in a guaranteed critical strike unless the weapon does not allow critical hits.

Gear

 * The most simple way is to invest in the Sneak perk tree, which automatically makes it more difficult for the character to be detected when sneaking.
 * The overall weight of armor affects the chances of being detected. Heavier armor is noisier, while lighter armor is less so. The Lighter Build and Ultra-Light Build armor mods can be used to reduce an armor's weight.
 * The Shadowed mod reduces the chances of the player being detected while in low light.
 * The Muffled mod on leg armor reduces the chances of the player being detected while moving.
 * Unfortunately Muffled and Lighter/Ultra-Light Build can't be used at the same time.
 * This isn't much of a problem with Leather armor, but it already becomes an issue with Raider and Metal armor.
 * Each U.S. Covert Operations Manual found provides a small but permanent sneak bonus.
 * Items like the Stealth Boy provide a large temporary sneak bonus.