If I upgraded from lets say Cubase 4 to 7,5 and one day I format my computer, do I will have to install 4 before or I can go strait with 7,5 ?
There is no difference between upgrading or buying a license for Cubase 7.5, i.e. either way makes a full license.
No, you would just install 7.5. As said above, you own a licence for 7 then you can install and run 7, doesn’t matter if you upgraded to it or bought it retail. And your licence for 7 entitles you to run any previous version of cubase as well.
My question was more speaking of files size… I f I buy the upgrade, is it the full package in size or just a patch to add to the old version.
If you buy the upgrade and have it shipped to you, your going to receive the full version of 7 on DVD with a free downloadable upgrade to 7.5. And if you download it, your going to be downloading the full version of 7.5. There is no upgrade that takes you directly from 4 to 7.5. Just did this myself upgrading from 6.5 to 7.5.