Firstly: Congratulations on winning the membership... woohoo!!

As for being unrealistic - it depends on a few things:

- do you have the time & energy to get to the classes, do the classes & return - without it disrupting your life too much?
- and as another said: you level of current fitness

If you are fairly sedate now - your body will be aching if you do 4 sessions in 5 days. And as you will know; water aerobics & yoga are not soft options! They can cause just as much muscle aches as a full on gym.

If you do too much too soon, your body will ache, and you won't enjoy it.
If you're already fit & do some regular exercise now - then go for it.
But if this will all be a shock to your body... I would suggest 3 sessions a week - alternate days... and build up to 4 sessions. Or three sessions one week, 4 sessions the following week.