It might be ok (it won't be 100% accurate, e.g. Fairchild's 7805 has a +-2% tolerance). Your 89s52 is probably ok at 4.7v.
However, it might be a symptom of having a poor power supply (e.g. drained battery, or less than 8v supply), or worse still,
your circuit may be consuming too much current (e.g. a fault). You can check that by seeing what the
current consumption is. Also, make sure there is a decoupling capacitor (e.g. 10-100nF ceramic) across the 89s52.
Also, make sure you've followed the 7805 datasheet, the capacitors that it will recommend are important.