Any suggestion(s) for an engine hop-up on my FXR4?
There's so much you can do to it. Give us an idea of what you're trying to accomplish and a budget, and you'll have a starting point. But without knowing that, the sky's the limit!
Hoist is right. I'd say to think about your ultimate goal and then start wading through the array of aftermarket parts. When I "did" mine, I thought about tradeoffs between performance and reliability, and cost versus actual performance gains, that sort of thing. There's always more that you can do but at some point most folks will ask "is it really worth it?".
Therre's a book called "101 Performance Projects for Harley Davidson Evolution Big Twins and Sportsters" that I highly recommend. It talks about all kinds of things and has a really good section on cam selection guidelines. Cylinder heads can be purchased from HD (SE) or S&S or Edelbrock and others or you can have the stock ones re-worked. Cost for each option becomes a factor versus results. SE heads for Evos can be had brand new via ebay for deep discounts.
Lots to think about. Bottom line--come up with a plan or goal, talk to folks who have been there, and figure out how much each step or component will cost versus payback in seat of the pants performance factors. And me, I figured in the CVO theme as part of the game (I started out looking at S&S heads but went with SE as part of the CVO theme. But I allowed myself to vary in cams because no SE cam had the specs I wanted, and so many Evo guys talked about Andrews.