The 4BD1T's came with a selection of different turbos depending on their year and original market.
Early (1985-1986) motors came with an oil cooled IHI-RHB6 turbo for which parts are becoming very rare.
The major diameters of both it's compressor and turbine wheels are 60mm. It is basically a japanese T3 size turbo. A workable turbo but not very efficient. It produces more backpressure for the same boost than later, more efficient, turbos.
It is wastegated, the exhaust housing scroll on my 1986 build engine has A/R of 11cm which is equivalent to 0.42 inches A/R. The wastegate should be factory set to approx 12psi.
The mounting flange on this turbo is split port with 8mm studs on a 73x35mm pattern. It has it's own high mount horizontal manifold. The mount pattern is not compatible with any other common turbo, but T2/T25 turbos can fit if the holes can be slotted.
A wastegated high mount T25 turbo was used on industrial 4BD1T and 4BG1T engines. The manifold is very similar to the top-mount IHI manifold but features a single port and T25 stud pattern (M8 studs on 73x40mm rectangle). The turbo used on the 4BG1T industrial was a MHI TD04HL-15T-12. This is a low boost turbo that will work in an automotive application but won't realise the performance potential.
Most T2/T25 turbos will bolt directly to this industrial T2/T25 manifold.
The later 4BD1T's (post 1988 update) and the early 4BD2T's had a non-wastegated (free floating) TB25. Several model codes exist including 466898 in Bush65's pictures below. It uses a T25 bearing core (water jacketed) and compressor, but has a T3 mounting flange which bolts to the factory manifold at an angle of approx 60 deg.
This non-wastegated (freefloating) turbo has a large turbine housing (0.87 inch A/R) and sized to run all day long in an NPR at ~3000rpm. It will not provide enough boost at 2000rpm for most swaps into smaller vehicles.
Some of the later 4BD2 engines (possibly 94+) have a wastegated version of the T25 turbo with a 54mm compressor wheel and 0.67 inch A/R turbine housing. Several model codes exist for these turbos including 465675 and 466409. The CHRA number 430425 on both. This is the best factory turbo and sufficient for mild performance increases over stock. The 4HE1 turbo (model code 700716) with a 60mm compressor wheel uses the same turbine (P/N 435257-0002) and may be a slot in upgrade. Keep the wastegated T3 turbine housing and slot in the 4HE1 turbo.
To gain more peformance from the 4BD1/4BD2 engines a turbo which provides boost earlier and provides more boost through the rpm range is desirable.
Many turbos exist from other engines and can be swapped in for a reasonable cost, but all require adaption to the intake manifold.