If I upgrade a firebird from a Mk3-8 to maximum Mk3 upgrade can I get Mk4
Fully upgrading a piece of equipment to be at full micro-upgrades for the current modification level allows you to purchase the next modification level at no cost, once you reach the rank needed to purchase that modification level. For example, if you have Mk4 Thunder and want it to be more powerful, you can fully micro-upgrade it, and once you reach the rank of Major, you can upgrade this to Mk5 at no charge, and start micro-upgrading from there.
The reason why the game doesn't just automatically give you the next modification level once you fully micro-upgrade something is because that would enable buyers/people with extremely large sums of crystals to have Mk7 equipment at Recruit rank, which would be completely unfair for battles.
Hope this helps!
Customer support service by UserEcho