Reopening closed tickets is possible but generally not encouraged. It can open to abuse either by staff but potentially for ‘tax reduction’ tactics
Changing payment totals can be achived in other ways by creating seperate adjustments. Simplest way would be to refund x$ and pay back same x$ with correct amount.
At hotel locally usually they would just use an open priced function menu product to do this but it happens less often nowadays and since we work from PMS accounts which i intergrated samba with I can adjust in that system since for them till is mainly used for item sales reporting rather than financial.
This forum is for SambaPOS 5 so it would be recommended you upgrade to V5. V4 is over 4 years old and not many people have experience with it anymore. Definitely you can implement a refund function in V5.