Bed size

So I have a 9 year old, 4 year old, and a 18 month old and I'm currently In the process of hopefully getting a new home and I've been looking into beds for my kids. I got one for my oldest but I'm wondering if I should get a twin or a toddler bed for my 4 year old and if I should put my 18 month old into a toddler bed or wait? Any advice would be greatly appreciated.. I know this sounds stupid but I'm terrible when it comes to this stuff lol..

My 4 year old is around 40 lbs and around 3 feet tall.. My 18 month old is about 25 lbs but short lol.