Incorrect, the 4.5L (1FZ-FE)is an Inline-6 configuration. Iron block, aluminum head and it never came with a turbo. The 2010 tundra/Landcruiser's new Diesel is a 4.5L and is a V config, with twin turbos too, was that what you were talking about?
Interesting we all speak of 22:1 compression ratio but if you'r running 9:1 (like me) and then run 22psi boost (like me) the actual compression ratio is over 22:1 when on boost.
On that basis the block/rotating assembly can sustain the compression.
Diesels need around 22:1 compresion ratio and direct injection to run.
I wouldn't even contemplate the engineering and cost involved to try it.
I seriously doubt it could be done.
Assuming it could be done I would think Toyota would have had a 4litre diesel V8. It would have been handy for them.