I'm assuming 4-5 is a subproof, as is 6-7. You should use some sort of indentation to do Fitch subproofs. I realize it's difficult here. I use multiple periods with spaces in-between (see here for an example) to indicate indentation.
1. The rule you're citing for Step 8 is incorrect. It should be OR elimination (which is the "By Cases" inference rule).
2. Step 8 is not technically valid as yet, but your proof can be made valid by just including two more lines inserted in the right place. Hint: try OR intro.