You would say "...y cuarto". "Las seis y cuarto" is "quarter after six"
If you mean the time: las cuatro y cuarto If you mean simply the numbers: cuatro, quince.
Son las cuatro y cuarto (It's 4 and a quarter) Son las cuatro y quince (It's 4:15)
Las seis y cuarenta y cinco. You could also say las siete menos cuarto. Or quince para las siete.
4:30
"Son las cinco y media." You could say, "Son las cinco y treinta" or "Son las seis menos treinta" but it is more common to use media to refer to the middle of the hour.
It's four minutes till ten. Also, some people say "to" or "of" in place of "till."
Cuarto in english is room
Four hundred in Spanish is "cuatrocientos".
"Son las once menos quince" or "son las once menos cuarto" (it is quarter to 11). You could also say "son las dies y cuartena y cinco" (which means it is 10:45). You could also add "de la mañana" (in the morning), "de la tarde" (in the afternoon), or "de la noche" (at night) to the end to make it more correct.
Son las cuatro y veinte
If you are trying to say las cuatro en punto it means four o'clock.