UKC

Looking for advice and experience new boot purchase for mont blanc and other snow and glacier routes up to 4000 in alps and up to 6000 in andes

Currently have boreal g1 which are warm but very heavy 

Go to a shop with experienced staff who know what they're talking about, and try some pairs on. 

Scarpa Phantom 6000 work for me up to about 6500m.

For the Huscaran etc most people i saw were wearing plastics.

As above I used Phantom 6000 in Peru and they were spot on for 6000ers. Last time I did Mont Blanc I used Charmoz.

I'm not sure you'll find one boot to do both; a 6000M boot will be unnecessarily heavy and cumbersome on a 4000M peak and a 4000M boot shan't be warm enough on a 6000M peak...