If it is truly a 1/4-acre pond, the 4 oz dose is the minimum rate that I'd suggest.
Application-volume (ie. how much water you mix with it) is somewhat irrelevant; but one should definitely use enough volume to adequately distribute the mixture throughout the pond's surface - or otherwise concentrate the "whole-pond dosage" within the infested areas (presumably the perimeter and/or along the down-wind shoreline).

2 gal of mixture over 1/4-acre (ie: 8 gpa total-volume equivalent) seems very light.
For future applications, I'd suggest using the minimum equivalent of at least 40 gpa (ie. 10 gal for .25 surface-acre).
Reasonably thorough distribution is important since this product may not remain "herbicidally active" for more than a few hours - especially at elevated pH levels. Although it isn't necessary to contact each watermeal plant for the treatment to prove effective, one still needs to use the "hand-grenade approach" (at least place the applications within close proximity of all targeted watermeal plants).

Kelly Duffie
Cypress, TX