During this season, the series switched over from using cel animation (the first few quarter-hour episodes were the last to use it) to digital ink and paint, Spencer Klein replaced Phillip Van Dyke as the voice of Arnold, and the opening theme was changed.
|#||Segment A||Segment B||U.S. air date||Production code|
|61||Eugene's Birthday||Stinky's Pumpkin||March 17, 1999|
|62||Dinner for Four||Phoebe Skips||March 31, 1999||161B /|
|63||Full Moon||Student Teacher||March 10, 1999|
|64||Big Gino||Jamie O in Love||March 15, 1999|
|65||The Beeper Queen||Oskar Can't Read?||March 29, 1999|
|66||Headless Cabbie||Friday the 13th||October 30, 1999||164A / 168A|
|67||Helga's Parrot||Chocolate Turtles||April 5, 1999|
|68||Love and Cheese||Weighing Harold||April 7, 1999||163A /|
|69||It Girl||Deconstructing Arnold||September 18, 1999|
|70||Grudge Match||Polishing Rhonda||September 25, 1999||173B / 170A|
|71||Veterans Day||November 6, 1999||169|
|72||Back to School||Egg Story||September 11, 1999||170B /|
|73||Weird Cousin||Baby Oskar||October 9, 1999|
|74||Helga Sleepwalks||Fighting Families||October 23, 1999|
|75||Monkeyman!||Buses, Bikes, and Subways||March 4, 2000||176A / 172B|
|76||Grandpa's Sister||Synchronized Swimming||October 16, 1999||176B /|
|77||Helga's Masquerade||Mr. Green Runs||March 11, 2000||177A /|
|78||Helga on the Couch||December 4, 1999||178|
|79||Dino Checks Out||December 11, 1999|
|80||Summer Love||June 21, 2000|
- This season consists of 20 half-hours, but only 17 were actually aired in the US during its 1999 run. Episodes 75, 77 and 80 were held over for unknown reasons and aired as part of Season 5 in 2000.
- Due to the change from traditional cel animation to digital paint, characters now have darker outlines and crisper, brighter colors.
- Hey Arnold! wasn't the only American animated series in the late 1990s-early 2000s to make a permanent switch from hand-painted cels to digital coloring. Other series that made the change include The Angry Beavers (1999), SpongeBob SquarePants (2000), Arthur (2001), The Powerpuff Girls (2001), The Simpsons (2002), King of the Hill (2003), and finally Ed, Edd n Eddy (2004).