Scooby - the maths is easier to figure out if you base it on how many people Virgin need on a single day to operate flights, rather than how many of the total workforce you'd be down regardless of whether they are scheduled to operate or not.
Depending on the day of the week, there's probably 28-ish return flights per day, with an average of 16 crew on each leg. That's about 900 people needed to cover one days' flights.
If, as you suggest, about 1000 crew are taking part in industrial action, that leaves 3000 to cover the schedules. Management will clearly do what they can to ensure as many non-striking staff are available for work as possible, and will schedule accordingly. Add to that the seconded crew outlined by Jcdcon, and it's a workable situation for Virgin.
Being 25% down on the workforce will be a pain; and I can quite easily envisage delays to flights because of the chaos of scheduling. But it could work, and if the strikes are limited to single days here and there, the company will ride through this.