Short answer: You're right that the 4 signifies a vase, and I'm reasonably sure that 1976 was the design year, with the last two numbers identifying the specific design.
Longer answer: You've hit upon one of life's great mysteries: Kosta Boda's numbering system. Or perhaps it's just me. I've read two versions: both say that the first of the five numbers (the system used between ca 1970 and 2002) represents the type of object (as you say). One source says the the next two numbers are the year of design and the last two numbers are the specific production number (Pina in Fire and Sea). Ivo Haanstra in Glass Fact File says that the last two numbers are the year of design. This would mean your vase was designed in 1938, the year in which Vallien was born. I don't think he was that precocious. None of the Kosta books I've got (Kosta 250, the catalogues for 1986 and 1994) explain the code system, although the overwhelming majority of the items pictured in the two catalogues seem to support the the idea that the second and third number give the year of design. Yet I've seen pieces where that doesn't make sense.
Sorry for a long response to a simple question; sometimes I can't help myself.