Increasing the Perception stat can increase your chance at a successful use of Pickpocket by approximately 2% per point. Heavier objects, such as most heavy armor pieces, incur a major penalty to your chances.
The heaviest equipped objects, notably the minigun and its energy variant, may not be pickpocketed as their weight reduces the chance to zero.
The maximum chance to pickpocket any object in Fallout 4 is 90%, regardless of any other perks, bonuses, or item weight modifiers.
Pickpocketing the fusion core from someone wearing power armor will cause them to exit the power armor.
In order to pickpocket equipped items, the player must be hidden from the target.
Uniforms and outfits can be "farmed" via Rank 4. So long as the target is stripped of all worn clothing (including armor), they will reset their outfit when the player re-enters the cell.
The game allows you to steal a synth uniform from certain synth NPCs in the Institute so long as they are not walking, such as when they are sweeping or acting as a merchant. These uniforms may be otherwise unobtainable even if the synth is killed.
Despite the wording from Rank 4 of this perk implying double the chance of a successful pickpocket, it appears the stealing chance is additive rather than multiplicative.