My name is George
Having said that your insulation have settled down i think you probably have a cellulose insulation, pay attention to the color if it is grayish looking then it's cellulose.
Four inches of settled down cellulose equals out to value of R13 ...so yes you should add more insulation to achieve a minimum of R30 in your attic.
I have attached a link to the manufacturer application coverage chart and for the value of R30 you would need a total of 9.2 inches.
Hope that helps and good luck with your project.