does anybody know what this rash could be that my 4 yr old has?

It could be heat rash. Sounds like heat rash. But i'd call the dr if your concerned.

Too hard to tell from what you say. Call the doc and give details on the rash. Take a temperature before you do. Worse case is an allergic reaction. Has she started new medication, new laundry detergent, new foods? Is it itchy, red, raised, what size are the areas of redness, when and what part of the body did they start. The intensity of the rash with crying probably isn't significant.

Take her to the doc, any rash deserves a doctors visit in my opinion, especially if it's all over her body. Could be anything from a viral infection ( roseola, German measles, strep throat rash), to eczema or mites. I would definitely take her in.