7805 regulator outputting 4V instead of 5V
If you are doing everything correctly then the chances of it working correctly are extremely high (99.999%)

SO it is very likely that you are doing something other than you say or imply.
Follow the following to see if we can find out what the problem is:

Are you using C1 and C2?
You MUST to start.

Are they close to the regulator?
Must be close to start.

What is your load?  

What is Vout with no load?
5V?

Have you put a new battery in your voltmeter ?
("Low battery" in meters is a too common cause of apparent voltage errors.)

What are you using for the PTC resistor?
If it is too high a resistance it will cause problems.

The PTC is wholly unnecessary to start and reasonably unnecessary in any case.
If you are using one, short it out.
Were you using one?

If the circuit is not your EXACT circuit please modify it to show EXACTLY what you are doing.

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