The sabbatical is a one off once per lifetime thing, you don't make the dedication roll for it you just elect to pay the money.
Yes, you do make a roll.
"After the general field has been chosen, the character must make a dedication die roll. Throw 8+, no DMs apply except when throwing to enter a physical fitness program ... "
This comes in the paragraph before the descriptions of the fields: education, weapon expertise, skill improvement, and physical fitness.
I don't see how it could be clearer that the 8+ roll applies to all of them.
If you don't get enough mustering out money, pension, tickets to sell to pay for it, or TAS membership (this is worth Cr60000 per year selling the tickets) then you can either do a few patron missions to get the cash or take out a loan.
And a 4 year sabbatical means the character is removed from play for 4 years. So the player has to either (1) sit out of the campaign for 4 game years or (2) generate another character to run while the first character is on sabbatical -- which rather completely defeats the purpose of sending a character on sabbatical.
This is an ideal way to separate characters from their money or to use as rewards from adventures, and it is guaranteed to succeed if you as the ref want it to.
It also guarantees players will quit the campaign or just abandon the first character and run another one.