It depends on the horse.
Some horses need every day riding to keep them focuses and listening.
Some horses you can jump on after a year of no riding, and it's like you rode them yesterday.
In general, younger horses need consistent riding at least 3 times a week, which makes sense because as they are learning, they need to continually build on their knowledge and training until you have a finished horse that is 100% broke to ride.
It's encouraging that she remembered all her ground work well, after time off. But no one can predict how much riding she is going to need to "maintain" her professinal training.
I will say though, it would be better for her as a 4-year-old to get some professional training now while she is younger, than if you just let her sit around for a couple more years doing nothing. Because then at least she'll have had something done.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.