You said it was too hot to spray or otherwise chemically treat your pasture; it would seem mowing is your only option for now. As others have noted, mowing is an important part (maybe the most important part) of controlling weeds.
When the time/temperature comes for chemicals, you might want to apply a Weed and Feed fertilizer. Granular treatments can be more effective due to their slow release, and they certainly are safer in terms of exposure to the person applying and later visitors. Additionally, Weed and Feeds give an extra boost to the grass while killing the weeds. A decent weed and feed from Home Depot would cost about $550 for four acres. Its application cost is certainly higher than just 2-4D, but it is also fertilizer and liquid sprays often require multiple applications due to their short lifespan (only impact weeds growing at the time of application).
I am reclaiming a 2.5 acre pasture. I applied Vigoro Weed and Feed and Spectracide Insecticide pellets with my Land Pride spreader in April of this year. So far, I have wiped out the fire ants (I sprinkled Amdro bait on a few survivors - not enough ants to make a mound) and greatly improved the grass to weed ratio. I was not expecting to be able to use my finish mower for a while, but I was able to use it for the entire field (minus a few rough areas and the perimeter) starting in May.