I would like to wire a 100amp subpanel located in an attached garage. I plan to use a 100amp breaker on the main panel for the supply. The wire will run though the attic ~30ft. I plan to run #4 copper in conduit unless recommended otherwise. I would appreciate any suggestions. Thanks. J

Do a search for "subpanel" in the Electrical Forum:

https://www.askmehelpdesk.com/search.php

For many details that may be involved with your situation.

You can come back with specific questions.

I'm primarily interested in whether I can wire the subpanel without using conduit from main panel. I would really appreciate some advice on the appropriate wire gauge and insulation type.

You can use #2-4 wire SER cable, which is aluminum, if allowed in your area. Most areas do allow this.