Very short answer
They are different generations of the same family of interplanetary spacecrafts.
Zond 2, as well as Zond 1 and -3; and Venera 2 and -3, were interplanetary spacecrafts (in Soviet/Russian classification this category falls under "automated interplanetary station") of 3MV family 3MV English Wikipedia page (in Russian "3МВ" 3МВ Russian Wikipedia page; Russian version contains more information on launches). 3MV stands for "3rd generation of Mars-Venus interplanetary spacecrafts".
Venera 4 was a derivative of Venera 3 (of 3MV series) spacecraft (and kind of "squeezed in between" 3MV and 4MV series).
Zond 2, as well as all other probes of 3MV series, was designed by OKB-1 under Korolyov guidance, Venera 4 design was assigned to NPO Lavochkin
The major differences between Venera 4 and its predecessor are described in this article (in Russian) from NPO Lavochkin webpage:
- Instrumentation compartment Temperature Control System was redesigned from scratch:
(all following quotes are translated from Russian)
Instead of using gas and liquid system, the specialists of the Design Bureau of the S. A. Lavochkin Engineering Plant redesigned the temperature control system to work purely on gas; this was simpler in production and more reliable in operation. In this case, the radiator – cooler was combined with the central part of the parabolic pointed antenna.
Total redesign of atmospheric entry module:
After the start of “Venera 3” the Academy of Sciences revised the model of the atmosphere of Venus and in the technical specifications for the [Venera 4] apparatus this has brought higher calculated values of temperature (425 ° C versus 330-350 ° C for “Venera 3”) and pressure (1-10 atmospheres versus 1.5-5) at the surface of the planet, therefore the [atmospheric entry/lander-impactor] module had to be re-designed.
[In order] to increase rigidity of the entry module, a gas pressure of 2 atmospheres was created inside the instrument compartment.
Electrical power supply system for onboard equipment/instrumentation of orbital module was substantially redesigned.
Substantial improvement of atmospheric entry module ablative heat shied:
Compared to Venera 3, thermal protection was significantly enhanced. To prevent heating of the apparatus during the descending phase, between the external thermal protection and the internal casing there was a multilayer thermal insulation made of fiberglass honeycombs with interlayers of asbestos textolite.
15% increase of the spacecraft mass:
The total mass of Venera 4 automatic station was 1106 kg. Since it was significantly larger in mass than Venera 3 (960 kg), instead of the 8K78 (Molniya) rocket the modernized 8K78M launcher (Molniya-M) was used instead.
P.S. The last mentioned article contains quite a bit more of technical information on Venera 4, although not explicitly stated whether those details were a change from Venera 3 or not. Although the page is written in Russian, google translate these days does a pretty good job.