According to Apple, 4.1 requires a Mac 512K. The 128k could go to System 3.2, Finder 5.3.

I would assume 4.1 needs more RAM, and bombs on a 128K (400K disk) or 128Ke (800K disk).

Were there ever any third party upgrades for the Mac 128K that took it to 256K? .

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I believe that System requirement had more to do with the 128K ROM than the RAM, as discovered when testing the highest System which will boot on a stock 512K.

Have you tried pairing down your 4.1 System on mini vMac? RAM may vey well be the culprit, but until the system is stripped it's hard to say for sure.

Tom Lee has basically said a 256K system is impossible, based on the way the System allocates RAM addresses.