I have always had kids sharing rooms, in various different arrangements. With 2 boys, and 2 girls of varying ages (currently 12, 10, 6 and 2).
Right now my boys are in separate rooms, and the girls share a room. Currently we have a single bed and a crib in there. When my youngest is ready, we will take the side off the crib and use it as a toddler bed for a year or so. Then, the plan is to have bunk beds. I wouldn't do bunk beds until the smallest kid is 3 or older (even if you plan to put them on the bottom bunk), but that might just be me.
Love all the options above. You could also do a loft bed for the oldest, and a regular for the youngest. Can do a bunk bed style set up, or move it around. We had a loft bed for my oldest daughter, and all her "stuff" was underneath, a little play area. But the little one kept climbing up there, and she was only 1 at the time.