2019 R3.4 is meant as a drop-in replacement for 2019 R3.3 and earlier releases.
However, stricly speaking, every bugfix and almost every improvement is a behavior change. For example, R3.4 improves client certificate selection logic in HttpRequest.ClientCertificates collection, which is a slight change of behavior. As another example, addition of CHACHA20_POLY1305 ciphers to TlsCipherSuite implies that applications using TlsCipherSuite.All will announce support for these ciphers after upgrading to R3.4.