4 If he were on earth, he would not be a priest, for there are already priests who offer the gifts prescribed by the law.

Read Hebrews 8:4 Using Other Translations

For if he were on earth, he should not be a priest, seeing that there are priests that offer gifts according to the law:
Now if he were on earth, he would not be a priest at all, since there are priests who offer gifts according to the law.
If he were here on earth, he would not even be a priest, since there already are priests who offer the gifts required by the law.