Trouble began on the sixth day of the flight, November 17. The capsule developed an atmospheric leak, the pressure first dropping from 760 to 380 mm of Mercury. With the drop in cabin pressure all the animal test subjects died. It would have killed any Cosmonaut not wearing a spacesuit.
The first attempt for a Zond 4 follow-up launched on April 22. It failed when the Launch Escape System sent an erroneous abort command at T+260 seconds and shut down the Proton booster’s second stage. The escape rocket fired and pulled the descent module to safety.
When we left the Soviet Union they had somewhat successfully landed a probe on Venus and they had completed the automatic docking of two Soyuz 7K-OK spacecrafts. However they did not reach their goal of a circumlunar flight in time for the 50th anniversary of the glorious revolution.
After 1957, the Soviets became accustomed to achieving “world firsts” in space accomplishments. Nevertheless, 10 years later they were not confident that they could pull off the world’s first fully automatic rendezvous and docking of two un-piloted Soyuz spacecraft. At the time the chance for success was estimated at only 50/50.