I want to extrude sketch 4 to be as thick as the rest of the part, but when I do I get an error message that says it will cause zero thickness geometry. How do I fix that?
Uncheck Merge result in sketch4's extrude.
..guessing... (have not looked at file)
Just a fair warning...
Unchecking Merge Results will create multibody parts (which could result in unwanted situations down the line of your design/modeling).
If you want it to be a single body, you will need to address the zero thickness geometry.
In looking at your part, you already have multiple bodies, without trying to add a feature from sketch 4. Is this what you really want?
Also by clicking the "troubleshooting tips" link on the error message, SW help loads:
EDIT: I would also suggest using convert entities or colinear mates to keep your edges together.
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