There are many rectifier tubes on the market and things can get confusing but we are here to help! The 5U4 is possibly the most used and famous rectifier of all time. There are many variants and equivalents for the 5U4, not all of them are drop-in substitutes though.
Drop-in Equivalents & Substitutes:
5U4GA, 5U4GB, 5AS4, CV1575, 5U4G (large ST tube - electrically equivalent but won't fit in some circuits)
This is a very popular rectifier to use in place of 5U4, 5AR4, 5Y3, 5V4 and many others. The 5R4 can handle high plate voltage and is one of the beefier rectifiers. It does have significant voltage drop so subbing a 5R4 will often result in a different tone.