Since Trezor Safe 3 was the 3rd gen wallet (as said in your marketing materials), the numbering was understandable (model one, model t, safe 3). Since you know released Trezor Safe 5, what is the meaning of 5, where is the Safe 4?
If you step into an elevator, sometimes the button for “13” will be missing due to superstition in the West. Likewise, in some Eastern countries the button for “4” wll be missing. In the Chinese tongue, the word for the number “four” sounds a lot like the word for “death”, differing only in the tonal inflection. (The word “four” has a “falling” tonal inflection while the word “death” has a falling and then rising tonal inflection.)
I think it’s likely the manufacturer chose to skip “4” for this reason.