Are you talking about the Haltbtax Plus?
If so, you can not get back the last bits of what remains of your budget. You could only get back what you paid into Halbtax Plus if you did not yet use up as much money as you initially put in. (Ex.: you paid 800.- for 1000.- of tickets. Then you buy 500.- worth of tickets. You can refund 300.- but the remaining 200.- are lost. If you buy total of 900.- worth of ticket, you can’t get back anything.)
You can use it up by buying a ticket worth more than 4 francs and paying the rest by a different payment method or you can refill your budget and take the remaining 4 francs into the new Halbtax Plus and continue to use it.