The manuals are all on the Mitel site but you must be a reseller to access them...
You will also need active SWA (SoftWare Assurance) with Mitel, and this would be a multi-step upgrade... I can't remember if you can go directly from 4.2 to 8.0, you might have to go to 6.0 first, but you have to get to 8.0, then do a migration from the old OS to Linux moving to 9.1, then you can upgrade to 9.4.
Honestly, this isn't something I would attempt as an end user... You should really engage your Mitel vendor as this is a fairly complex task with many steps and could vary depending on any other services you have like MiCollab, Teleworker, Call Recording, Call Center Management, etc. and it will involve a fair amount of downtime and prep work.
