There are infinitely many numbers between 0.4 and 0.5, but I'd imagine what you're asking is "what number is halfway between 0.4 and 0.5?"
Remember that trailing zeroes (the zeroes after the end of a decimal fraction) don't alter the number itself, so we can also write 0.4 and 0.5 as 0.40 and 0.50, 40 hundredths and 50 hundredths. Since 45 is halfway between 40 and 50, it makes sense that 45 hundredths - 0.45 in decimal form - is halfway between 40 and 50 hundredths.
