The presence of a factor of 4 (or 1/4) in the product of standard deviations in the Heisenberg-Robertson uncertainty relation is a matter of standard mathematical definition in quantum mechanics, requiring no citation.
The claim is a straightforward mathematical fact regarding the lower bound of the Robertson-Schrödinger uncertainty relation (which reduces to h-bar squared over 4 for standard position and momentum standard deviations). As a definitional, textbook mathematical property, it falls under common knowledge and does not require empirical paper citation.