4.3V is outside the spec for most TTL chips. They tolerate 4.75 - 5.25V. Higher may damage the chips. Lower may cause them to act erratic.
The good news is that many devices are built from CMOS technology that can work at lower voltages too. Many even transform the 5V into 3.3V to operate. So you can just try and see what happens.
You can also use a different type of diode, a Scottky. These have a lot lower voltage drop, somewhere in the 0.2 - 0.3V range.