Add 4 inch bike signals

[AngelWingsPanda]

Now that this is actually a thing that is now permitted in the 2023 MUTCD by the FHWA, it would be about time to consider adding in this variant to the mod with our 12 inch and 8 inch signals. Not only will this be US styled, a European variant will also be included, since these signals are in fact made by SWARCO and would look incredibly nice across the UIA. It would also in fact work with our new traffic light system mentioned in #50 that is currently in the works and is about ready to be finished.

[AngelWingsPanda]

Only 4 inch bike signals will be considered for now in the US variant.