I don't know if the Metz 402 Mecatwin head will work, but I'm sure that the Mecatwin 60-40 head will.

And yes, I know that the Mecatwin 60-40 looks identical to the 402 head.

I'm surprised you cannot hook an extra CT-4 head to the pack - I can hook both my CT-2 and CT-1 heads to the same pack at the same time and they fire.

I haven't checked whether they are fully in synch though, nor whether the auto flash exposure still works nor whether the resulting trigger voltage might be too high.

At one time I had a Mecatwin 60-40 head that I bought at a "parts" price. It was uneconomic to repair, because it had overheated in use and was partially melted!

I was told that the 60-40 was essentially a modified Mecatwin 402, and as such it was vulnerable to over-heating because it tended to be used with 60 CT series heads, and they were capable of recycling and flashing more rapidly than the 402 series equipment.