This is the cover to Newton Comics' The Avengers #4:
It originally appeared on the cover of Marvel Comics' The Avengers #5:
The story contents are as follows:
The Invasion of the Lava Men!, originally published in The Avengers #5, My 1964
Trapped by the Red Barbarian, originally published in Tales of Suspense #42, June 1963
Both of these stories are reprinted without undue editorial modifications based on my comparisons to reprints in Marvel Masterworks and Essentials editions.
This issue contains a Captain America poster:
This is modified from the cover to Marvel Comics' Captain America #109:
Note the Marvel branding has been replaced by Newton branding in the poster.
This poster would be repurposed by Newton as a Free King Size Super-Hero Poster in 1976.
This issue includes in-house advertisements for Conan the Barbarian #1 and Doctor Strange #1. They are both advertised as On Sale Now, and are both listed on the subscription page.
As mentioned re: The Avengers #3, to this stage The Avengers was being published every three weeks. The Avengers #3 was published 9 August 1975, so The Avengers #4 would have been scheduled for publication 30 August 1975.
However, in response to a fan letter in this issue, John Corneille advises that The Avengers "appears fortnightly from now on." This is ambiguous - it could mean effective immediately, or from next issue onwards. Given there is a two week gap between the contest expiry dates in The Avengers #3 and #4 - 22 September and 6 October respectively - it firmly suggests the fortnightly schedule was effective with this issue. I have cross-checked this with subsequent issues and I am satisfied this is correct.