...
WHERE
x_field IN ('f', 'p', 'i', 'a') ...
ORDER BY
CASE x_field
WHEN 'f' THEN 1
WHEN 'p' THEN 2
WHEN 'i' THEN 3
WHEN 'a' THEN 4
ELSE 5 --needed only is no IN clause above. eg when = 'b'
END, id
...
WHERE
x_field IN ('f', 'p', 'i', 'a') ...
ORDER BY
CASE x_field
WHEN 'f' THEN 1
WHEN 'p' THEN 2
WHEN 'i' THEN 3
WHEN 'a' THEN 4
ELSE 5 --needed only is no IN clause above. eg when = 'b'
END, id