What is this yellow capacitor?
Yellow axial caps like that are (almost) invariably polyester film (for which Mylar is a trade name). They have very low leakage and fairly mediocre dielectric absorption performance. 

The part number seems more like a 4uF/100V than a 15uF. Unless the OPs fingers are huge the size would better fit as well. The other types of film cap that are relatively common (PP and PC and PS) would be much bigger again. Stick in on a capacitance meter and check it. Put 100V across it and verify it doesn't fail. There's not much that can go wrong with a cap like that.   


  PP = Polypropylene (good DA performance) 
  
  PC = Polycarbonate
  
  PS = Polystyrene (would be huge even in 4uF) 


There are more modern films such as PPS but they are uncommon in that style of capacitor. 

As to where you would use such a part- it might be used for an analog filter or an analog controller. Much of this kind of thing is done in the digital domain these days, but there are still applications.

Your bypass application would probably be better served by a ceramic capacitor (perhaps in parallel with a low-ESR aluminum electrolytic)- lower inductance, smaller and cheaper most likely (depending on voltage). The film caps do have longer lifetime than electrolytics.