Ukrainian, not Russian

Oddly, the translation algorithm actually realized that this construction means "is four years old" - though not "is ONLY four" as the всього makes it mean. But it doesn't really understand what's happening in the grammar. If it did, it would know that the noun whose age is being given is in the dative. The word for "stamp" is марка (marka). Марко (Marko) is a very common boy's name. The word here is Марку (Marku) - that's the dative for the name. The dative for a stamp is марці (martsi).

Also, this is Ukrainian, not Russian (it sort of knows that - it gets a lot of the Ukrainian words right, like чотири,  хлопчик, врятував, and справжні. But it got tripped up in a few other places. От (ot) is not the Russian preposition "away from" but rather an adverb/interjection meaning something like "lo, voila, here, you see" (it's вот (vot), in Russian). And while рыбалка in Russian is a fishing trip, or the act of fishing, in Ukrainian the word рибалка (rybalka) means "fisherman" (рыбак (rybak) in Russian). And lastly, in Russian the dash is often the copula, in Ukrainian it rarely is; Ukrainians use the verb much, much more often that Russians do.

I would probably translate the last sentence more like "Real heroes do live among us" or "There are real heroes living among us", but that's a stylistic quibble,  not a grammatical or semantic error.

"Mark is only four, but he's already a serious (= genuine) lifeguard. But you can see he loves ice cream like all children. Together with his mom Daria the boy saved a fisherman from the water. They live among us - real heroes."