No, a 4 blade is not suitable to be used in a 4.5 angle grinder. First, they are quite small to be used in a 4.5 angle grinder and also can wear out very quickly. Since the blade is smaller, it works at higher speeds and that’s why you will have to replace it sooner than usual. Also, smaller discs will not be suitable for any grinder of a larger size. Not only will you have to keep replacing it very quickly and it will cost a lot, but also it won’t give you efficiency in your work. You will only end up wasting your time, money, and electricity, in the process.