Yes Twenty Minutes four times of Week of exercise on a Stationary bike is a pretty good workout to help keep you fit and healthy.
Although if you can you should go for 15 minutes per day on the stationary bike and do it at least 5 days of the week for much better health results and to keep your heart healthy.
I can't stay on a stationary bike for 15 minutes at a time but I do it in intervals throughout the day.
What I do is get on the bike and ride it for 5 minutes and then get off and take a break and then again get on the stationary bike and do another 5 minutes and then repeat again for another 5 minutes which equals 15 minutes minutes per day.
Walking around town is also a great exercise so if you can get out and walk at least 15 to 20 minutes each day and if possible walk to places instead of taking the car as your body will be much healthier and you increase your chances of living longer and preventing heart disease, strokes and heart attacks.