So the last time I shot my No4 Mk1 I noticed that it was shooting further and further to the left. Turns out the front sight will drift under finger pressure. Is it time for a new blade, and if so, where is the best place to get one?

Depending on time and place of manufacture, your rifle's front sight base may have a set screw at the front. If so, tighten it and see if that helps. If it does not, or it still moves, you can peen the edge of the blade's dovetail and/or the base to get a tighter fit.

No4 front sight has two variations of the block, three protectors and four blades styles, any combination could be called original.

Most all the front sight blocks have a screw at the front that tightens the dove tail.  Inverse slot head.  I use a 3/16 allen tip and cut a slot in it.  The war time expedient base does not have the screw, found mostly on Long Branch and Savage, most have been removed from service over the years.

If yours does not have a screw you can peen the dove tail with a center punch.

Front blades are found on evil bay.

Just a simple dovetail. Looks like I've got some peening to do.

The blades used in the solid foresight block are supposed to have a cut at the bottom to act as relief when pushed into the slot.  If you have a blade without a cut, that may be the problem.

It does have a cut.