4" will not work if you want to keep the ducting that runs from the bottom of the glovebox to the small vertical vent that faces toward the passengers, or between the fuse box and the steering column on the other side. Crutchfield specs the 3.5", but you have to remove those two ducts.
While those ducts do not serve much purpose, I hate taking things out at random.
If 3.5 or 4" are all you can fit in there, I may just forgo those and do the ones in the doors.

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TripleJack

85 GenI SWB 2.5/5-Speed on 33s

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