まっ四角[masshikaku] square
一天四海[ittenshikai] the world
下四半期[shimoshihanki] last quarter (of the year)
弦楽四重奏曲[gengakushijuusoukyoku] string quartet
再三再四[saisansaishi] repeatedly

三寒四温[sankanshion] alternation of three cold and four warm days
四つ[yottsu] four
四つんばい[yotsunbai] (crawl) on all fours
四つ角[yotsukado] four corners
四つ足[yotsuashi] four-footed (animal)

グリーン車[guriinsha] green car (1st class)
愛車[aisha] (one's) beloved car
医療車[iryousha] clinic car
井戸車[idoguruma] well pulley
１等車[ittousha] first-class carriage (coach)

一輪車[ichirinsha] unicycle
横車[yokoguruma] perverseness
下り列車[kudariressha] trains going away from the capital
下車[gesha] alighting (from train, bus, etc.)
火の車[hinokuruma] fiery chariot (which carries the souls of sinners into hell)

ダイヤの指輪[daiyanoyubiwa] diamond ring
一次線輪[ichijisenrin] primary coil
一輪[ichirin] one flower
一輪車[ichirinsha] unicycle
花輪[hanawa] wreath

競輪[keirin] keirin
金輪際[konrinzai] the deepest bottom of the earth
九輪[kurin] kurin (nine vertically stacked rings of a pagoda finial)
結婚指輪[kekkonyubiwa] wedding ring
月の輪[tsukinowa] moon (esp. full moon)