Class 1 includes gross earnings in CASH or assets readily convertible into cash – note that on the business mileage allowance it is ONLY payments in excess of the approved 45p per mile rate that are chargeable.
Class 4 is based on the tax adjusted trading profits after deducting any trading losses.
May I suggest that if you want additional detailed knowledge then I would recommend that you buy a study manual from on of the approved providers which will provide you with as much detail on any area that you may want – enjoy!