I have a 3 to 4 cm red bump with a scab in the middle

If it is reddened and it has a constant scab that develops on it, have a physician have a look at it. Has it become bigger since last year?  What color is the scab?  Is it an area that is exposed to the sun a lot?  I am uncertain what to tell you what it is by not seeing it, but if it is red, it could be something that needs to be treated.  Does it bleed when you remove the scab?  It may be nothing if there is no bleeding to it.  I have a 1 cm area on my shin that doesn't go away and it's been there for over two years.  It just more like scar-tissue than anything.  If you decide to go to the doctor, let me know what he/she says.