https://pmc.ncbi.nlm.nih.gov/articles/PMC8658652/
> Recent studies indicate that E4 is an estrogen with a distinctive profile of ERα activation. E4 activates the nuclear ERα, but it is an antagonist of the membrane ERα, in contrast to other estrogens [14,15,16]. Based on its pharmacological profile, E4 can be classified as the first Natural Estrogen with Selective Action in Tissues (NEST) [17]. NEST activities of E4 are the consequence of its unique dual role.
> On breast tumor tissue it acts as an estrogen antagonist in the presence of E2 [21,22]. The estrogen-antagonistic effect of E4 in the breast has been further supported by a recent pre-clinical study that has been performed in women with breast cancer, finding that E4 reduces breast cancer cells proliferation [21,23,24,25,26]. These features could suggest a future role of E4 as a selective estrogen receptor modulator (SERM), but with less adverse effect than tamoxifen (hot flushes, nausea, hypertension, thromboembolic events, endometrial hyperplasia)