IME, two inches of rain is ideal after application. I guess that four is fine also. You need feet of rain to really leach a soil.

And I may disagree with that. I expect that 4 inches of rain is enough to flush much of your fertilizer away.

I had the same sort of question over on Tomato forum. Some plants look like Mg deficiency after 2 weeks and 7+ inches but soil tested high in Mg before. I'm wondering if all the rain (or low N caused by rain) is inhibiting take up of other nutrients? Would blood meal (pure N) help or is a balanced fertilizer (Neptune's Harvest) better?

Yes, four inches of rain in a soil that drains will leach away anything that doesn't latch onto the clay or organic matter. Things as leachable as nitrate nitrogen will mostly be carried well below the surface foot.

To be honest I'm not to sure low levels of N is going to affect uptake of nutrients in the soil as the plant itself will be trying to uptake what it needs I do know that some of your micro nutrients do aid in the metabolism of the plant and its up take of macro nutrients I believe zinc is one of the important ones the only way to really tell is time if there is a deficiency the plant will show it then u have to identify and amend the soil don't worry as you can apply micro nutrients as a foliar spray for a instant boost as well as amending the soil if you can find them in liquid form

It probably depends on the OM%. My empirical experience is that with 21% OM, clay soil, the fertilizer is not leached by two inches. I also expect that leaching will be an exponential function of precipitation, that is, if x inches leach 50%, 2x inches will leach 75%.