Wouldn't spay a family pet at 4 months. Too young. Six months is pretty common. Some owners hold off awhile and wait to watch the baby tooth situation. Also, there are some questions about bone development and hormones that make owners hold off. I'd personally spay a Yorkie close to 6 months.
The ovariohysterectomy (OHE) is done in the US. Removal of uterus and ovaries.. The (what I assume is termed) ovarectomy (OE) is done in Europe and is just a removal of the ovaries. Honestly, there isn't much difference long-term between the two. This is something you could Google and see the pros and cons of both. Ellie's vet does the OHE. No reason to leave a big organ behind that can get a tumor.
There is an anesthetic protocol thread in the YT Library. That would be good to go over.
Baby teeth will change color as pup gets older because they are dead (as the adult teeth come in). If the adult teeth are turning yellow, it could be because of the baby teeth right there, but you may need to get them cleaned if it continues. Some vets will do it during spay and some won't. It's done last in case the pup is unstable and has to be woken up. It's a good time to start brushing daily.
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