flowers for small bed in lawn

Instead of flowers, why not go for foliage colour which lasts a lot longer? There are many different heucheras which would be suitable

For such a centerpiece bed, you could plant taller plants in the center, medium height next and maybe annuals around the outer rim. That gives you the ability to change annual colors without a lot of work, keeping the foliage color year round.

And also plant small bulbs like Iris reticulata, Crocus and Scillas or Chionodoxa for spring colour. Don't plant Daffodils or Tulips because you have to wait until their leaves have gone over and they tend to look a mess in a small bed. How about a small evergreen shrub in the centre for winter interest - or a dwarf Phormium? You could plant Cyclamen around it to brighten your winter days.

I love my pittosporum 'Tom Thumb' evergreen small shrub...dark purple-y foliage, with contrast of new green leaves. Other pittosporums with green/silver leaves. You would want to plant little smaller things around it like Spritzhenry suggests...mini cyclamen, snowdrops, English bluebells, some aquilegia when it's bigger perhaps? Don't know how large your bed is, but mine's next to a compact lavender which looks nice, a penstemmon & a veronica
...ah just re-read your question, 4ft ok