I have filled the same crack four times and it keeps coming back, is this the wrong filler or the wrong diagnosis?

Four failures in the same line strongly suggests the crack is moving, and no filler survives movement. So diagnose before you buy anything.

What to check:

Is it seasonal? Cracks that open in winter and close in summer are usually normal movement - timber drying, the building breathing. Common and not structural.

Does it follow a line in the structure? Along a joist, at a ceiling junction, above a door, at the join between two materials. Those are movement joints in all but name and they will always crack.

Is it getting wider over time? Put pencil marks either side, dated, and measure monthly. Stable width means movement you can accommodate. Steadily widening is a different conversation and worth a professional.

Is it stepped or diagonal from a corner? That is a different category and not a filler question.

Assuming it is ordinary movement, the fix is not a harder filler: it is a flexible one, and reinforcement. A crack that moves wants either a flexible decorator's filler that can stretch a little, or scrim tape and a proper patch that spreads the movement over a wider area instead of concentrating it on one line.

Rigid filler in a moving crack is guaranteed to fail, and it fails exactly the way you describe.

The lime and gypsum question is real and it is a different problem from yours, though worth understanding because it will come up.

Old solid walls have no damp proof course and are designed to breathe. Moisture gets in and evaporates back out. Lime plaster is porous and allows that; it also handles small movements because it is softer than the wall.

Gypsum and cement are much less permeable and much harder. Put them on a solid wall and two things happen over time: moisture that used to evaporate through the surface is trapped and goes somewhere else, and the hard patch does not move with the soft wall around it, so it debonds at the edges.

The classic failure looks like a patch that stays perfect in the middle and cracks in a neat rectangle around its border, sometimes with damp appearing next to it rather than in it.

That is not what you describe: same line, four times, no damp - so I would treat yours as movement first.

But the general rule is worth adopting anyway: on an old solid wall, match what is there. If the existing plaster is lime, patch with lime. Small filler repairs in gypsum are usually tolerated; large areas are where it bites.

How to tell what you have: lime is softer, scratches with a fingernail, and is often on timber laths rather than on the brick.

Practically, for the repair itself, since technique accounts for a lot of repeat failures too.

Open the crack out rather than filling the visible line. Rake it out to a slight V so the filler has a key and some depth. Filling a hairline gives you a two millimetre skin with nothing holding it, which fails the first time anything moves.

Remove all the loose material and dust, then dampen the edges before filling. Dry old plaster sucks the water out of the filler immediately and it cures badly.

Fill in two passes, slightly proud, and cut back rather than trying to get it flush in one go.

Use scrim tape across the line if it has failed repeatedly. Bedding a strip of mesh in and skimming over spreads any movement across a wider area instead of concentrating it.

Let it dry properly before painting. Weeks rather than days for anything deep, and a mist coat first.

And a realistic expectation: in an old house some cracks come back regardless. If it is stable in width and only a cosmetic annoyance, a flexible filler and a repaint every few years is a legitimate answer rather than a failure. Not everything in a hundred year old wall is meant to be permanently solved.