noun
semifinalists; final four
Refers to the four teams or players remaining in a tournament after the quarterfinals. Used mainly in sports contexts.
日本代表は見事に４強入りを果たした。
The Japanese national team successfully made it to the final four.
ワールドカップの４強が出そろった。
The World Cup semifinalists have all been decided.
準決勝 means 'semifinal' (the match), while ４強 refers to the four semifinalist teams or players themselves.
ベスト４ is a more general term for 'top four' in any ranking, while ４強 is specifically used for tournament brackets.
Compound of ４ (four) and 強 (strong), literally 'the strong four'. A relatively modern term used in sports journalism.