You can get a 5500w heater too, just FYI. Not sure if you're limited to 4500w. As for safety - first and foremost, I'm not a professional electrician so I may not be correct, but...
The breaker at your main house panel and wiring to your dryer should be matched, IE the breaker is sized to trip before the wiring melts. The rating you read off the breaker is the maximum power you can draw from the panel, so stay under that (some recommend 80%, it's well debated on this forum, decide what you want to do). The plug & cord you build should be able to handle whatever power draw your panel will pull - so do the math and reference the charts to see what wire gauge you need. You'll need to run a 4 wire (3+1 / H-H-N-G) cord. The spa panel serves as GFCI protection almost exclusively, so it being 50a even though you're not pulling 50a isn't a problem - the breaker at the main will take that role. The spa panel should be somewhere in the cord you build, after the outlet for the dryer and before your panel. You can do without the spa panel if you replace the breakers in your main panel with GFCI breakers, but it generally works out that it's faster and significnatly cheaper to just install the spa panel. Then, once the wiring makes it to your brewery panel, you can install smaller breakers to allow you to run smaller wire to individual components with increased safety and overload protection.
