I'm a bit of an herbicide nerd - as I've been spraying deer foodplots for many years.
Generally, I try to understand what plants i'm targeting... if its "everything" thats easy... glyphosate (roundup) is cheap and effective, especially with a surfactant added in.
In your case, sounds like you want to hammer the broadleaf weeds and leave behind the grasses. There are several herbicides that'll work. Most of them are likely to contain 2,4D or 2,4DB.
Between those two, I'd look for 2,4D as it is more broad spectrum effective against broadleaf plants. (2,4DB is more picky about which broadleaf weeds it'll kill). Most big box hardware stores will sell herbicides that should fit your need since people have the same need in their lawns.
One last comment - if you're spraying 4.5 acres, I assume you're not out there doing it by hand - and are instead using a spray trailer or similar larger tank setup. Just watch that 2,4D and 2,4DB around trees, especially if using a boomless sprayer - wind drift can wipe out trees you didnt intend to kill... ask me how I know