"I cannot verify if any of them are pregnant or not," Mats Brännström, head of the research team at Sweden's Gothenburg University, told AFP.
"We will wait until late pregnancy to release any data on that."
The embryos – produced by in-vitro fertilization – were transferred into the transplanted wombs during the past month.
The researchers said "the prospects for success are good", but they did not expect all four women to get pregnant on the first attempt.
"In some patients we will surely have to repeat the procedure several times until we get a good pregnancy," Brännström said.
The patients are part of a group of nine who had received transplants of wombs donated by close relatives since September 2012.
In two cases, the transplant was unsuccessful and the womb had to be removed.
Another three women are expected to receive embryos in late March or early April.
The world's first transplant of a uterus, donated by a 46-year-old to a 26-year-old, took place in Saudi Arabia in 2002.
Although blood clots forced doctors to remove the transplanted organ after 14 weeks, they claimed technical success.