Our current glyphs for the numbers have evolved in Europe from about the tenth century onward when they were adopted from Arabic in Spain. (Before that they have made a whole development from their partial Indian origins that I will not get into here.) Here is a nice website that describes the changes the glyph have made through the first centuries of the second millennium. Note that Wikipedia is a nice starting source for research on specific numbers, each number has a separate subsection for the development of the glyph (see for [2](https://en.wikipedia.org/wiki/2_(number)#Origins) and 4.

The two has evolved from two parallel stripes that were connected. Until around the thirteenth century it was actually written upside down as compared to what we do now. The little loop at the end seems to pop up now and then, but may be just a matter of taste or convenience.

The four is a different matter. It has had a variety of forms, usually based on a cross. In Europe two legs of that cross were connected to form something like the ribbon that is used in campaigns against AIDS (red) and breast cancer (pink). This then evolved into the closed 4 we know now. Opening up the top (while preserving the diagonal) may have been related to distinguishing it from the 9 in handwriting, in print I have not encountered it until more recently. The only thing I know about the full open 4 is that it is used on digital displays of watches in 1970's and 80's. I would suppose the open 4 in fonts and hand writing is derived from these uses. Funnily enough the open 4 looks more like the ancient and medieval predecessors of our glyph for 5, which was a lower case h rotated 180 degrees.