Couple of questions:
1. Which 4000? There are two - a lovely 1970s one with single cassette deck based around a simplified version of the old Beocord 5000 and a Beomaster 2200 - and the 80s very sleek but ultimately Japanese twin cassette affair. Quite an interesting machine and the only twin cassette sold by B&O but actually made in Japan.
2. Does the audio work OK playing cassettes and the radio?
Surprised you have a MMC10E as I thought these were a US model. Soundsmith sell new replacements. I doubt the cables have anything wrong with them. Maplins do sell new plugs and cable though, including right angle DIN plugs which the old 4000 benefits from.