|This article is about a/an ranger in Choudenshi Bioman.|
- ―Yellow4's roll call[src]
Mika Koizumi (小泉 ミカ Koizumi Mika) was the original Yellow4 (初代イエローフォー Iero Fo Shodai) of the Biomen. She was a photographer who dreamed of following in her brother's footsteps to photograph the wildlife in Africa.
Initially, she was hesitant to join the Bioman team. However, she was convinced when she realized that if she did not fight, then even the animals in Africa that she loved would eventually be in danger. For someone who was hesitant to do battle, Mika proved a powerful and skilled martial artist.
Ultimately, Mika heroically took it upon herself to give her life to save her teammates, and sustained several repetitive blasts of Anti-Bio Particles from the Bio Killer Gun wielded by Mason. Following the complete depletion of her Bio Particles, Mika was then hit with powerful attacks from Psygorn, resulting in her death. The team later gave her a proper funeral while still in her Yellow4 form.
Mika is avenged when the Biomen destroy Psygorn, and Silva destroys Mason.
She is avenged again when the Biomen finally defeated Doctor Man.
In light of Mika's death, the Biomen were forced to search for someone who could take her place as Yellow4. They came across a female archer named Jun Yabuki, and Peebo, recognizing that she possessed Bio Particles as well, recruited her to the Bioman team.
In the last episode, Peebo apparently forgot to recall her for some reason in the script though Peebo remembers her during the first appearance of Silver.
The footage for Bioman seen in the first episode of Turboranger ends up using clips that ultimately show both Yellow Four: the original Mika Koizumi and her replacement Jun Yabuki. This is the only compilation with multiple Sentai that show both Yellow Four from Bioman.
At the end of Hikonin Sentai Akibaranger: Season Two, Yellow Four I appears with two other deceased Super Sentai warriors, Time Fire and Abare Killer, before the deceased Akagi, Yumeria, and Luna at Himitsukichi. After the Akibarangers bid farewell to Hiroyo and Kozukozu, they disappear along with the three Sentai warrior ghosts.
- Mika Koizumi/Jun Yabuki/Yellow Four: to be added
Super Sentai Legend Wars
Yellow4 is among the vast pantheon of Rangers who feature in the mobile game Super Sentai Legend Wars.
Mika is a bit of a loner at times and can be a bit stubborn, although she has a good heart it is shown despite her attitude at times Mika is loyal to the rest of the team even if at times she doesn't show it. She also has the habit of taking pictures at the weirdest times (such as when Biorobo grabbed her in the first episode) and also appears to be a bit of a UFO enthusiast.
- Biojet 2 (shared with Green and Blue)
- Super Electronic Holography
- Yellow Flash
- Mika is the first female Yellow Ranger.
- Her English dub name is Casey, which is also the name of the Red Ranger from Power Rangers Jungle Fury.
- She is also the first female Sentai Warrior to die.
- For Mika's last episode, her voice was dubbed over by an uncredited Mayumi Tanaka as Yuki Yajima had only completed ADR for the first nine episodes before leaving abruptly.
- With Mika appearing as Yellow4 in her Akibaranger cameo, it confirms that she is still considered a Ranger even after death, even with her replacement as Yellow4 in all later appearances including the Legend War.
- Near the end of episode 10, Bio Robo's eyes glowed triggering her Bio Hologram to feature a footage of herself. The footage shown is her shot in the closing sequence.
- Jun Yabuki - The Sentai who replaced her as Yellow4.
- Trini Kwan - The first female Yellow Ranger and first female Yellow Power Ranger to be replaced.
- Kendrix Morgan - First female Power Ranger to die while active as a ranger.
- Trini Crystal - Power Rangers counterpart in Bio-Man. See comparison page.