The main cast has now added Dylan Everett (Derek Davenport) and Jeremy Kent Jackson (Douglas Davenport). Will Forte will also play a recurring role of Daniel Davenport, who is Douglas and Donald's older brother who lives in Hawaii. (Will Forte also voices Eddy, and as revealed in "The Secret Weapon: Part 1", Eddy was given his voice because both Donald and Daniel created him together.)
Kelli Berglund (Bree Davenport) had left the show prior to Season 4, because in the Season 3 Finale, she went to Australia because of her scholarship there. However, Bree is now a recurring character on the show.
In the fourth season, Donald reveals that he and Douglas weren't the only kids in the family. They had an older genius brother named Daniel (nicknamed Danny just like Donald and Douglas were nicknamed Donny and Dougy) who currently resides in Hawaii! When Daniel calls one day to ask about him, Donald tells him that Douglas is alive and that a man named Victor Krane is planning to take over the world with his bionic soldier. Daniel reveals that he knows Krane, and that he is truly evil. Because of this, he sends them his only son, Derek, via transporter, so that Donald could upgrade him to become the ultimate 14-year-old bionic machine! Now, together, Adam, Chase, and Leo must help their new leader master his powers... but worst of all, he must controls his glitches too, so that he can become the Lab Rat's "secret weapon"! But elsewhere, Krane is more powerful then ever since he has reprogrammed Marcus (after the accident) to become bionic, and not an android. However, Marcus does not have new powers and only his old ones. But when Krane finds out about Derek, he upgrades (or should I say "steals, and then upgrades!") his own "secret weapon", one that could be more powerful and experienced than Derek himself!
Will Forte as Daniel Davenport
TBA as Josh Foster
Stefanie Scott as Sarah
Zendaya as Natalie Beige
Dylan Everett as Flame