Array Formula with nested IF/THEN in average - how to ignore zeros

I have the following spreadsheet. I created an average column which needs to calculate an average on the following conditions:

-Numbers are computed in average
-Values beginning with an "<" are divided by 2, then computed in average
-Cells containing "--" are not counted in average

How can I get the average while ignoring the "--" cells? This is how I have it set up now:

{=AVERAGE(IF(ISNUMBER(B4:G4),B4:G4,VALUE(RIGHT(B4:G4,LEN(B4:G4)-2))/2))}

Any suggestions or help on how to make this formula work?

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Thank you Bo, this worked great. I added a way to count the number of cells in the range so I could use this for any size set of data:

=(SUM(B5:G5)+SUM(IFERROR((LEFT(B5:G5)="<")*MID(B5:G5,2,9)/2,)))/((ROWS(B5:G5)*COLUMNS(B5:G5))-COUNTIF(B5:G5,"--")))