4 hours should be fine to get it to temp. If you want to get the texture changes then you will have to go long, really long and low. Something like 24 hours at a lower temperature. I don't know how low though.
I would try searing in a very hot cast iron skillet for 2 to 3 minutes per side then into 375F oven for 15 minutes or so till done the way you like. other option would be into oven at 500F for 10 minutes then turn oven off and let sit for 30 minutes without opening the door.
It will stay as good as traditional rare in a low sous vide. In Fahrenheit you need to keep it at or above 131 for a long safe cook. My brains not awake enough to do the conversion to centigrade. 129 is what my computer says for 54. By the standards here in the US you shouldn't cook below 131 for more than an hour as you're in the danger zone for bacterial growth. 131 will achieve pasteurization with the longer cooking time and remain pink and juicy via sous vide.
Ok that's great! 131°F is 55°C so that's what I'll do. Looking at some other recipes I think 6-8 hours should be sufficient. I'll put some aromatics in with it, then I'll flash sear it in a big pan and rest it before I serve.
At the time/temp you are doing you will achieve an even cook all the way through, but will have done nothing to enhance the tenderness. If you are OK with how tender a sirloin roast is, then by all means do as you plan. I'm not being snarky (sometimes hard to tell on internet so I'm clarifying). I often find sirloins to be a bit tough, but certainly edible just cooked until done...as with another cooking method such as oven roast or grill. Again, you'll be perfectly cooked but won't help tenderize.
You could increase the cook time several hours to enhance the tenderness. I would think 12 hours MINIMUM would work, but you would probably see noticeable results in tenderness more like 24 hours. This is what I do for homemade "roast beef" style sandwiches, etc.
I would do 2 things in addition: I would season with lots of salt and pepper at least a day ahead of time. This will greatly enhance the flavor of the beef and allow the seasoning to penetrate the meat.
I would also sear the outside of the meat BEFORE vac sealing and sous viding. This will enhance the safety by killing bacteria on the outside of the meat. You will and should also sear again after the meat comes out of the bag.
Another thing to consider is to cut down your sirloin so that you have 2 roasts (smaller) and they will cook faster and be more manageable. This is up to you but it will cook/tenderize faster with no loss of quality. It just depends on what your final product will look like.
I did my large piece of Sirloin for 8 hours in the end (it's what I had available. This gave me a fairly good mix - it was mostly rare, but moved to medium for some of the chunks (55ºC seems to hot for rare in my experience), but with the aromatics, it was very tasty.
I'd recommend my Anova wholeheartedly, especially if you get some custom plastic containers from SousVideTools. It's currently chugging away doing some pork belly at 70º (which I cured first) in a cool box, should be ready for searing when I get home!
Vacuum machine wise, I've got a food sealer I bought from Costco. Great for Sous Vide and freezing things, however I wish I'd spent a bit more getting a chamber vacuum sealer, as then a world of compressed foods, sealing liquids well, and marinating better would be open to me! These start at around £350.
For simple cooking, I get good results with a freezer grade zip locking bags. Load it carefully, pressed out excess air. Seal all but a corner. Slowly slip it into the bath letting the water compress the bag and force out the remaining air. Seal the last corner bit right at the end.