You don't necessarily a weak mix, but weaker than the bricks around it as it is easier to replace the pointing than it is to replace a brick.
Think about this: if you're repointing, surely the wall has been there a few years.
When the bricks were initially laid, they didn't do a different mix for pointing.
Besides, if there's any excessive movement, no matter what you do, bricks will crack and pointing will do as well, whether it's a soft or hard mix.
So, just do what has been done forever, 4:1 mix.
Most importantly is the preparation that counts.
Remove old mortar at least to 20mm deep.
I go 25mm.
I use a grinding disc on a 115mm grinder to remove most of it, then a Irwin mortar rake kit set at 25mm to clean it all up.
Hoover the lot and I even use compressed air for best results, but that's a bit over the top, I know.
Then dampen the area and compact the pointing properly, making sure you get a smooth finish.
Then for the next 50-100 years, depending on weather, it won't be a problem anymore.