Getting correct WKT result to 4 decimal places using PostGIS?

The simplest way to round geometry coordinates is to use ST_SnapToGrid:

SELECT ST_AsText(ST_SnapToGrid('POINT(-0.685239554498251 51.4940418030595)', 0.0001));
       st_astext
-----------------------
 POINT(-0.6852 51.494)

But if you just need the 4 bounding coordinates, then ST_Extent returns a simple box2d type, which sort-of looks like WKT:

SELECT ST_Extent(
  ST_SnapToGrid(
    ST_Transform(geom, '+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +datum=OSGB36 +units=m +no_defs', 4326),
    0.0001))
FROM (
  -- whatever geometry table goes here, e.g. two points
  SELECT 'POINT(123 4567)'::geometry geom
  UNION SELECT 'POINT(8910 11121314)'
) s;
               st_extent
---------------------------------------
 BOX(-15.9517 49.8078,-7.5602 89.9958)

(You may also want to assign an SRID for that projection in your spatial_ref_sys table, as ST_Transform performs faster using geometries with these set)

And if Mike T's answer isn't what you want, you can get the X and Y coordinates out with something like

SELECT 
    round(ST_X('POINT(-0.685239554498251 51.4940418030595)'), 4),
    round(ST_Y('POINT(-0.685239554498251 51.4940418030595)'), 4);