In doing the assessment of grade 4 and 5, please remember that in case of grade 4 some resistance should be applied at the end of the range. While in case of grade 5 full resistance should be applied at the end of the range. Resistance should not be applied throughout the range of motion as it will lead to a faulty assessment.
Further more, I'm also attaching here a concise review of manual muscle testing and muscle examination charts as an attachment here. Have a look over that dear.