Kids do gross things sometimes.
My main concern with the question and follow-up is that you are TTC again when you are having such issues with your current littles (lack of sleep- yours and theirs, stressors, etc.) I know tone can't be conveyed in text, but please believe me that I am coming from a place of concern and not judgment. You will get even LESS sleep during pregnancy and w/a newborn, and while that is temporary, that year or two of focusing on those things might hurt your current relationships w/your kids (not to mention DH, if you are super stressed/exhausted all the time) might do a lot of long-term damage.
It sounds like you are on the right track w/getting her some oral stimulation toys. Four year olds still nurse in most of the world, so being very oral at this age is nothing to be concerned about, and some people just are more than others (errrr... smokers? ;) )
There are great websites out there w/toys that aren't super expensive- I love Sensory Edge, and eBay and Amazon can be good too. I have some items stashed in my purse, the car, and in the kitchen for times when my kids start chewing/licking.
Sorry if I repeated anything- I didn't go through and read the responses. Hopefully you will get some peace with things soon.