If Japanese developed 4 engine bomber on the base of Kawanishi H8K flying boat in the same way as Short developed Stirling bomber on the wing of Sunderland flying boat they could have it already in 1942.
By the way Kawanishi had partnership relations with Short company.
Such Kawanishi G5K 4 engine bomber could look like as on pic below
Characteristics could be even better than Liberator D had an in general close to Emily data.