I have extruded a sketch and am trying to join it to other sheet metal sections but am having difficulty doing this. There are 4 sections in total and 3 of them I have been able to merge by converting an extruded surface to sheet metal and defining the bend edges. The last sections am not able to do this, due to lack of experience on my part.

Attached is the model and am trying to get 'base extruded 4' to join with the rest of the sheet metal parts in the model.

Any tips would be greatly appriceated.