From the floorplan it looks like a loft conversion could work, but it depends on your roof height and structure. As a rule of thumb, you need at least 2.2m of head height in the loft at the highest point for it to be practical. Most people in your situation go for a dormer, which would give you the headroom for a good-sized double plus an ensuite.
If you need four bedrooms upstairs, the loft is usually the most straightforward option compared to reconfiguring the first floor. The key is whether the existing loft space is tall enough and whether the staircase can be fitted without eating too much into your current bedrooms.
Best bet is to get a couple of builders or loft specialists round to check what’s possible. You can also compare rough loft conversion costs for your type of house.