Some less-overt G4 references I could make out on the standard Netflix release of My Little Pony: A New Generation! I believe there are more to find, but I can't see everything in the regular resolution.

1. Illustrated page visible on the lighthouse beacon room table, featuring a drawing from the Tale of Two Sisters opening of the G4 pilot.

2. Segment of Twilight's stained glass window, specifically the version seen in Princess Twilight Sparkle and not the movie, also on the same table.

3. Fossilized imprint on Argyle's desk that seems to match up with Spike's tracks.

4. Illustration of the Mane 6's stained glass window seen in Return of Harmony hanging up on the pinboard.

5. Bottom half of one of Zecora's masks visible hanging on the wall of Argyle's office.

6. Book page on the pinboard featuring the Memory Stone artifact that was banished to the human realm, and most notably seen in the Equestria Girls: Forgotten Friendship special (now THAT one's pretty niche!).

7. Portrait of Starswirl the Bearded on the floor by the book stacks, again in the office.

8. Grogar's Bewitching Bell on a pile of books nearby.

9. Another illustration on the pinboard of the Tree of Harmony.

10. When Zipp shows Sunny and Izzy the abandoned station, the timetable for departures and arrivals can briefly be seen. Maretime Bay, Bridlewood and Zephyr Heights are listed, as are Neighagra and Manehattan which were seen in the G4 series.