How high is rate of conception if 3 vs 4 embryos transferred into never-pregnant 42 year old from proven donor?

Too high!: The rate will depend on the donor's age. Transferring 1 or 2 embryos will avoid the risk of a large multiple pregnancy with its many hazards to you.

Depends on donor age: One or two embryos is the right number to transfer from a typical egg donor under the age of 30. Three embryos would be unusual but may be acceptable if the donor is older. If we have to transfer 4 embryos the donor is not an acceptable candidate to be a donor in my opinion. Hard to give numbers without knowing the donor's age, but a 75-80%+ chance of success with 2 embryos from an under 30 donor.

ASRM guidelines: I agree with above. Asrm guidelines steer towards 1 or 2 embryos with a young proven donor and no other negative prognostic factors. I tend not to direct my patients towards eset, but that's just me. You could consider transfer of 3 under extenuating circumstances; failed prior ivf might be one. Four embryos is pretty far outside of what is becoming, like it or not, a standard of care.

Not a good idea: An egg donor produces embryos with high implantation rates, in the range of 50-70 percent. Transferring 2 embryos with a 70 percent implantation rate creates a twin risk close to 50 percent. (0.7x0.7 = 0.49 = 49 percent). It is bad strategy to transfer 3-4 donor embryos to a recipient.