Your builder is wrong to say its fine if the mortar is crumbling away.
The mix sounds ok at 4/1 if this is what he did use, the problem is either the sand is to soft, and maybe should have been sharper, or I suspect he done it during the low temperatures we had, and it has killed the mortar, which I try to avoid, or never used a wintermix/frost proofer or covered his work up at the end of the day.
Another problem could be that he back bedded the tiles one day, then came back to point up over, which isnt good practice.
I would suggest you got your builder back, rake out all the mortar and rebed and point up in one, if he is a decent builder Im sure he will put it right.

A 1:4 mix is fairly strong mix which should be fine however it shouldnt be sandy and from what you have described i doubt it is 1:4.As for the cracking the tiles if the mix was stronger i think the builder having you in the bag.

There are few possibiltys why pionting can be damaged in this way
1. It has been dried too fast by the sun
2. Or it has been caught by the frost
As for mortar for brickwork and pointing anything from 3to1 to 6to1 is ok. though 4to1 is the preferd mix

As other roofers have said, the best mix for ridge is 3 to 1 mix. Pointing up tends to look ok but will dry and crumble within no time at all. Its always better to reseat rigde tiles.

personaly i would of used a 3/1 mix. hope it all goes well for you

I use a 1-3 mix with a pva admixture and a plasticiser in my roof mortar mixing. The pva bonds everything together. I think the "crack the tiles" comment is nonsense. I have been roofing for years and never heard of such happening.

hi,when repointing i always gauge the mix.using a large mug so you get the same colour and consistancy.are you sure he took out all the old mortor and never went over the top etc.i certainly would investicate and get it done properly has this may cause problems.if your happy get him back and explain to him you need it doing again and ask him how he mixed up etc.many thanks kentishbuilder

Sounds to me as if the mix was wrong (4:1) should be strong enough. It could also have been not mixed right, used to dry of the pointing has just been skimmed over.

Hope this helps.

all depends on the type of roof you have but usually the mix for ridge tiles would be best in 3 to 1 mix if the property is in a very exposed location

soory but i only use 2-1 mix with plasticiser the other mixes seem to week

If your builder has used a 1:4 mix he must have done something terrabley wrong, you are within your rights to be conserned, it shouldnt be crumbleing away with your fingers, a 1:4 mix would look rather strong anyway so i cant understand why it would look sandy, just tell him your not happy with the standard of work hes done you.

wrong mix somewhere along the line, fingers? to crumble it? rake out and replace

I agree with that of the other builders Three-In-One mix is the best way forward and repointed ridge tiles is really not correct you should taken off and reset them on correctly