It is puzzling, why T-IV in the 2003 only and WS thereafter. Dealerships I spoke to, use Bulk WS in all not calling for Dexron. They use bulk Dexron when called for (100 series 98-02), unless transmission running hot, which they then they use Mobil 1 MV full synthetic ATF in place of bulk Dexron. Most shop don't even carry T-IV or use it.
I use:
Mobil 1 98-02
T-IV 03
WS 04 up.
I feel 03 will handle WS just fine, for that matter I feel Mobil 1 could be used in all. But I'm not taking the chance with any transmission "yet".
Interestingly (Exxon Mobil) Mobil 1 sell their full synthetic WS, but not in the USA. Exxon also make most all Toyota lubes.