Hello Mike and welcome to the forum! This is a years-old thread, but as good as anywhere for your question. I've got 1978 LSIs, and while I've not heard the Forte IV, I can't imagine giving up the LSI's. I went through mine completely, tightening, replacing caps and a woofer, and generally cleaned them up. If I were you, I'd do the same and perhaps redo the cabinet exterior. You'd have essentially new LSIs at lower cost than the F-IVs.
Charlie