So firstely, you got your arch made out of 4 brushs.

Then you make all of these brushes as displacements.

Copy the result 2 times like below

top view

Select only the faces below the arch and subdivide them

Then delete the displacements you copied and keep the middle one

Now to smooth the sides, i'll use the same method. Copy the arch 2 times like before, and adjust the copied arches with the vertex tool

Then subdivide only the sides

Select and delete the arches you copied and keep the one in the middle, like before ..

And here you go :o