In the serial, the First Doctor (William Hartnell) and his travelling companions Vicki (Maureen O'Brien) and Steven (Peter Purves) arrive on an arid planet, where they encounter the beautiful but dangerous Drahvins and the hideous but friendly Rills, two crash-landed species in conflict with one another.

Both species wish to escape as the planet is set to explode in two dawns, but the Drahvin leader Maaga (Stephanie Bidmead) wants only her people to make it out alive.

Mervyn Pinfield was originally assigned to direct the story but his failing health shortly into production prevented him from continuing, and he was replaced by Derek Martinus, a new director.

The huge and impressive, horned warthog-like Rill explains that they have offered to take the Drahvins away with them but Maaga has refused, preferring to maintain a state of war.

[1] Spooner commissioned Emms to write the serial,[16] then named Doctor Who and the Chumblies on 1 March 1965, with a script due date set for 15 April.

[24] Galaxy 4 was Pinfield's final work for Doctor Who and the BBC, though he remained uncredited on the broadcast version; he retired from active television production shortly thereafter.

A ring of small domes in the shape of rocket engine nozzles surrounded the base of each robot, hiding the casters on which the costume moved as well as imitating the propulsion units of the mechanoid.

[24] The Drahvins were originally named Dravians, and written as a race of male soldiers; during casting,[2] which took place in early June 1965,[21] Lambert suggested that their gender be switched to female,[2] in part as a nod to the emerging women's liberation movement and to better emphasise their attractiveness.

[22] A photocall for the Chumblies and Drahvins took place at Ealing on 24 June;[29] this was also Martinus's first day on the programme, when Hartnell, O'Brien, and Purves were released from rehearsals of the third episode of the preceding serial, The Time Meddler (1965), to film inserts.

Martinus had planned for long tracking shots in the first episode to indicate the expanse of the set, but the camera crew convinced him to focus primarily on Hartnell, as viewers were mostly interested in his performance.

[5] The cast had troubles during the rehearsal process for the serial due to transitions within the production staff: Purves was upset following Spooner's departure as he had developed much of Steven's character, and Hartnell's relationship with Wiles began with difficulties; O'Brien helped Hartnell through his struggles with the production, and Purves began developing a friendship with him, inviting him to dinner every fortnight.

[32] The serial's final scene with Jackson as astronaut Garvey was filmed alongside the following story, "Mission to the Unknown", on 6 August 1965, and inserted into Galaxy 4 during editing.

[37] At the Missing Believed Wiped event on 11 December 2011, it was announced that the third episode had been discovered earlier that year among materials owned by former television engineer Terry Burnett,[34][13] who had purchased it at a school fete in the 1980s but not realised its significance until speaking to Ralph Montagu, head of heritage at Radio Times;[38][39] the final shots and closing credits were missing from the episode,[34] and the tapes featured some visual discrepancies that required cleaning.

[38] After the broadcast of the second episode, Bill Edmund of Television Today wrote that the characters were "a little sluggish after their holiday", praising Vicki and Steven's replacement of Ian and Barbara but describing the story as "rather slow".

In The Discontinuity Guide (1995), Paul Cornell, Martin Day, and Keith Topping felt that the serial "presents an interesting if flawed twist on the traditional bug-eyed monster tale".

[42] In The Television Companion (1998), David J. Howe and Stephen James Walker described the story as "an unfailingly entertaining one", praising its original ideas and high production value.

[43] In A Critical History of Doctor Who (1999), John Kenneth Muir called the serial "intelligent", largely due to its unique philosophy of making the humanoids "monstrous" and the "ugly" characters friendly.

[44] In 2012, Radio Times reviewer Patrick Mulkern thought that the serial was "by no means a classic" but did have "sparks of originality", particularly in Lambert's changes to the Drahvins and Martinus's dramatic camera angles.

[45] In 2021, Starburst's Paul Mount criticised the story's writing and "lifeless performances", though noted that Bidmead "manages to imbue her character with a bit of pseudo-Shakespearean gravitas".

Using off-screen photographs, animation, and audio recordings, an abridged reconstruction of Galaxy 4 was included in the Special Edition DVD release of The Aztecs in March 2013, alongside the full recovered third episode.

The animation is viewable in either black-and-white or colour, and the release includes documentaries on the serial and its recovery, audio commentaries, and remastered versions of the surviving clip and episode.

[51] Starburst reviewer Paul Mount praised the release's special features, but described the animation as "half-hearted" and noted that it was "likely to be left on the shelf to gather dust".