Two aspects to that question.
1) It's been suggested that when a new owner takes over a franchise, they should be given an option to either retain the previous owner's budget (with the +/- 4 adjustment), or just get "reset" to 10 across the board (as it currently works now). Seems to be a reasonable suggestion.
2) The reason why there is a +/- 4 adjustment limit is to prevent owners from gaming the system, particularly with scouting. For instance, one season you can bring your advanced scouting up to $20m, cut-and-paste everybody's projections into a spreadsheet, and then bump the advanced scouting budget back down to 0 for the next couple of seasons. It also prevents owners from doing the same with their draft and/or IFA scouting budgets from season to season, i.e. one season deciding to go all in on IFA and punt the draft, or vice-versa.