If it was yesterday, forget about trying to denature the toxin, it has been absorbed long ago and now you have the residual effect. Just ice the area, and watch it to make sure (you said it was about 4" already which is pretty big...) you don't get cellulitis from it. Watch the red area to make sure it does not get too big, it will feel very hot to the touch as well.
The welt (wheal) is probably warm to the touch already, just watch it to make sure it does not keep getting larger. Cellulitis will have to be treated by a doctor.