I connected 5V from 7805 to the 5V on the Arduino
Why would you do this? The Arduino has it's own 5V v.reg.
First off, I would disconnect anything connected to the Arduino headers. Then if you're still
measuring only 4.3V on the 5V rail, with either USB or power to the Power Jack [assuming
you have an adequate external supply], it sounds like you have damaged the v.reg, or less
likely some other parts on the board. Short of knowing how to go through and troubleshoot
every part, sounds like time to buy a new one. [we all do it].