my say- -unless there is a danger in waiting for the 5th or some benefit from cashing at 4, the "default" is: hold on until you get 5. but a lot depends on the spoils you are talking about and the particular risk reward possibilities at the time,.
e.g. With escalating spoils many wait until the fifth, figuring a bigger cash if they wait a round but that does not always happen. And if you finish off your turn with 5 spoils you can be a tempting target to anyone who sees any chance to eliminate you, grab your 5 spoils, cash, and then its off to the races. Compare that to how you would be seen if you cashed, had a strong troop holding after your turn was over, and only one spoil for someone to try to grab.
With flat rate spoils the default may be to hold out for a rainbow set rather than cashing a red, green or blue threesome, but Is the possibility for later gain worth by-passing the immediate gains possible by cashing now, maybe gaining a strategic advantage, say taking or breaking a tert zone boni? .
and so on and so on
![Very Happy :D](./images/smilies/icon_e_biggrin.gif)