Inoue, who became the first-ever 2-time Shooto world champion, was awarded the unanimous decision by scores of 30-29, 30-29 and 30-28.
Inoue improved his record to 15-3 while Kadowaki fell to 13-8-2.
Fighting for his fifth time in 2008, Hatsu Hioki (17-3-2) remained undefeated for the year by ground and pounding his way towards a TKO win over Rumina Sato at 3:32 of the first round.
Sato, whose record fell to 24-13-2, has now lost three straight and five out of his last fights, but to his credit, the losses were all against high-quality opponents in Hioki, Kadowaki, Akitoshi Tamura, Antonio Carvalho and Gilbert Melendez.
"Shooto Tradition 4" results:
1. Nobuhiro Hayakawa and Yuta Nezu went to a majority draw
2. Taisuke Okuno def. Daisuke Okumiya via unanimous decision
3. Atsushi Takeuchi def. Katsuya Murofushi via submission (RNC) - R2 (1:44)
4. Yutaka Ueda def. Jin Kazeta via TKO - R1 (3:23)
5. So Tazawa and Hiromasa Ogikubo went to a majority draw
6. Takashi Nakakura def. Bendy Casimir via submission (RNC) - R1 (4:58)
7. Hatsu Hioki def. Rumina Sato via TKO (strikes) - R1 (3:32)
8. Takeshi "Lion" Inoue def. Hideki Kadowaki unanimous decision