the JMSDF purchased tech for Soryu from kockums of sweden (is that a subsidiary of HDW now?). Kockums was also the vendor designer for the 4000t collins class. if you want a 4000t high endurance SSK design they have the experience and hard lessons learnt.
if we want a 1500t design then HDW U214, Spain Navantia S1000, French DCN "Marlin" , Amur are all there....problem is these might not be suitable for the long range strike missions in south china sea....a reason why australia chose a uniquely large collins design was to patrol their vast northern seas. to fight the fight you have to get there and get back which is where I think these uber-large SSKs do better.
to my knowledge other than Aus and Japan nobody has a design or plans for 4000t SSK. even the old Kilo is quite large compared to the smallish SSKs being sold these days.
so we might talk to both Japan (for sub fabrication and other aspects) and to Kockums (for all aspects). one adv is these designs carry 30 weapons in torpedo room vs 14 in kilo/scorpene types. in an era where less has to do more, a surviving large sub can keep up the fight for much longer using its better endurance and higher weapons count....not a SSN by any means...10-15 knots slower submerged for fast breaking transits, lesser weapons (SSNs like seawolf have room for 60-80

) , more austere sensor pkg (no cost-no-object spherical bow sonar)...but bigger legs and teeth than then ambush SSKs generally available.