As discussed in the previous post, step 4 of Social Security's sequential evaluation process is a determination of your ability to retun to your past relevant work. Your "past relevant work" is work you have performed in the past 15 years at the SGA level.
All Social Security does at this step is compare your physical and mental residual functional capacity to the requirements of your past work.
To determine the requirements of your past work, SSA looks at the work history report, which describes how you actually performed your past work. SSA also considers how the job is generally performed in the national economy, according to the Dictionary of Occupational Titles.
Social Security looks at the fit between your retained work capacity (your RFC) and the requirements of your past work. If you still retain the capacity to perform your past relevant work, your claim will be denied.
For more on this topic, read Social Security Ruling 82-61.