What's the difference between quadrennial and quadrennium?

Quadrennial

Definition:

(a.) Comprising four years; as, a quadrennial period.
(a.) Occurring once in four years, or at the end of every four years; as, quadrennial games.

Example Sentences:

(1) One of its functions is organizing a quadrennial international cancer congress.
(2) The Perugia Division of Cancer Research (DCR) owes much to HLS because he was always ready with advice and help of every nature and because the Perugia Quadrennial International Conferences on Cancer (PQICC) had their beginning through his will and always enjoyed his authoritative approval and aid.
(3) But in most countries where watching the game of football is a regular occurrence rather than a quadrennial diversion, they understand – unlike Americans – that its purpose is to incite and, in part, appease the bloodlust of the disenfranchised masses.
(4) Quinn's real chance came when the Board in 1985 gave her a new assignment: updating ICN's history, the result being ICN--Past and Present, which was launched at the 19th Quadrennial Congress in Seoul in 1989.
(5) By 2009, the equivalent stage of the next quadrennial World Cup cycle, that figure had climbed to more than $1bn.
(6) The authors delivered this paper at one of the Special Interest sessions in Tel Aviv at the Quadrennial Congress and stimulated the audience to much thought.
(7) Australia is being assessed before the universal periodic review , a quadrennial assessment of countries’ human rights record by the UN human rights council.
(8) She was also the keynote speaker at ICN's 19th Quadrennial Congress in Seoul, Korea, 28 May-2 June 1989, described by Korean Prime Minister Kang Young Hoon in his address at the opening ceremonies as the "festival of peace and friendship for nurses the world over".
(9) In my opinion, this equates to a possible situation where only a tiny minority of the most gifted, talented and motivated children compete in a single quadrennial competition.
(10) The pattern for 2016 looks very similar to 2015, but with the bonus of the maxi-quadrennial events of the visually stunning Rio Olympics, the Uefa Euro football championships and, of course, the US presidential election to boost marketing investments, as usual by up to 1% or so,” he said.
(11) The quadrennial survey continues to provide useful information on an easily identifiable and traceable patient population, but the process would be greatly simplified by the adoption of a "universal" reporting system such as that used in Europe.
(12) The flaws highlighted by Clinton in the country’s energy infrastructure, including pipeline spills, rail car explosions, and the exposure to cyber-attacks, mirror the findings of the first-ever quadrennial energy review conducted by the Obama administration and released in April.