Rapamycin inhibits mTOR which reduces the creation of new cells. For neutrophils with a short lifespan of under a day you will see that issue quickly. Platelets are affected the same way, but the result is seen in a reduction of platelets perhaps a week or 10 days after taking rapamycin.
Because I take rapamycin no more frequently than every 6 weeks and I have weekly blood tests you can see the platelet count going down quite a bit a week after taking rapamycin.
FIB-4 is a test used to indicate whether fibrosis of the liver is present.
As you can see from the above formula the platelet count is in the denominator. Hence if platelets go down FIB-4 goes up. FIB-4 is not that reliable for people over 65 and under 35 as well.
However, it is worth being aware that any shift in platelet numbers from mTOR inhibition should not be seen as the presence of liver fibrosis.