The worst thing you can do is weigh yourself outside of your chosen weigh day and time.
Your weight will fluctuate up to 7lb in a single day, so if you've decided to use e.g. Thursday 6pm as your weekly WI point, getting on the scales on Monday at 7am won't give you an accurate reading.
It's easy to say, but harder to do, but stay away from your scales except for your chosen WI time.
And 4.5lb in a week is bloody amazing!! Don't feel that it's not good, because a lot of people generally only lose up to 2lb each week. 4.5lb is fantastic!!!