16 inch

I would personally do a 16 inch bike, unless you have a very tall child who is shortwaisted but has long legs. I think going from 12 to 18 inch is too big a gap. Kids bikes are pretty cheap, and with spring and summer coming up. You can probably get a good deal on one at 16 inches that she may use just for a year or two before moving up if she is very tall.
There are also 14 inch bikes if you want to baby step it but sounds like she could be ready for 16. (If she still uses training wheels it may be hard to find ones that for 16 in). Make sure she is proficient riding her 12 in without training wheels before siding up .

Probably 16 inch, some 4 year olds may need a 14 inch

I would go to or call the bike store and ask them- the local shops give very helpful advice. I gave my kid's inseam and height and skill level and they steered me towards the right size.

I am planning to get a bike for almost 4 year old girl, and she is 40 inches tall now. She has 12 inch size bike, and it seems too small. She can manage to ride her brother's 18 inches size bike, and he is 48 inches tall for 7 year old.

I am confused if I should get her 16 inches bike, or another 18 inches bike?