Thunderbird 4 is a part of the Carlton toy company electronic boxed set. While this vehicle does not make any sounds itself, the others in the set each play phrases from the show. This line of Thunderbirds toys are somewhat new, being released in 2000.
If you where like me, you where first introduced to the Thunderbirds through Dinky Toys, a die cast toy line that was imported to the U.S. from England. Thunderbird 2 was one of many English properties that made the rounds, and Thunderbird 4 was included with number 2. Now it would be a little while before I would actually see the show (It would air here back in the day about 5:00 AM at first, later it would move to 6:00 AM and then in the evening during a children's cartoon show), but being a kid with a pretty active imagination, I always thought Thunderbird 4 was a shuttle craft , and Thunderbird 2 was the mother ship. I was also kind of confused as to why they were not called Thunderbird 1 and Thunderbird 2, being they were the only two that were released then.
Well upon seeing the show I learned that thunderbird 4 was actually a submarine used by International Rescue for all it's ocean rescue missions. The craft is one of a number of vehicles carried by Thunderbird 2 into action. I guess it was the most used out of all the little crafts that it actually received the Thunderbird designation and a number.
The Dinky toy version of this craft was far less detailed, so I was happy to find that this Thunderbird 4 actually looked like it's television counterpart.