I use old Nuun/Hammer tubes.
I’m riding a 1500km audax this weekend, London-Edinburgh-London, so this topic is ultra relevant to me. I’m planning to fuel with 60g/hour of my homemade drink mix, plus eating real food at most if not every control point.
That means a total of nearly 4kg of drink mix powder for me… Fortunately this event has 2 drop bag points, so quite easy to manage.
I bought these bags from ebay, putting 120g of my powder mix into each, and then bagging up again in re-usable silicone food storage bags for safety. I’ll carry a maximum of 8 at a time, and mix one 120g bag in one of my 750ml bottles for each ~50km stretch, keeping the other bottle for pure water.
I would normally just use small, reuseable silicone ziplock food bags for everything, but for this event the scale and weight means disposable makes sense.
I use old High5 hydration tab tubes and/or Berocca tab tubes 
Big fan of this tiny dry bag: Sea to Summit Lightweight First Aid Dry Bag for Medical Supplies Amazon.com
Can carry enough to do 15ish hour ride, reusable, durable, gets smaller as you use the powder, and does not leak.
I found that silicon slashers break open, can’t carry enough plastic nuun/film canisters and don’t like the dead air after I use. Zip locks break.
6oz refillable travel bottles: Amazon.com: humangear GoToob+ (XL) | Refillable Silicone Travel Bottle | Locking Lid | Food-Safe Material | Clear, XL (6 fl.oz.; 177ml) : Beauty & Personal Care
Those are wide mouth, so easy to fill though you still need a funnel. Twist off the cap and it dumps neatly into a bottle. Holds right around ~135g of my homebrew approximation of tailwind. Fits nicely in a bar bag or jersey pocket, super easy to clean and dry. Only downside, IMO, is that they’re bulky compared to a single use bag.
I’ve been using the “race tubes” from Flow Formulas, although they have their downsides.
- The lid can be opened by squeezing the tube. Hasn’t happened yet, but I imagine if enough pressure is put on it you could end up with powder everywhere.
- They take up quite a bit of space, so it’s annoying once you’re done with it you still have to find a place for the tube.
Otherwise, I like them because they are reusable.
For 4 hour rides, I’d probably consider just bringing more carbs in the form of a higher concentration in the water bottle, and just drink less of it throughout the day. Or, go with making a highly concentrated gel in one of those reusable bottles. The plastic tubes work well in a pinch though. I usually put a bit of tape on the lid.
I’m on team super concentrated bottles. I do up to 300g of Gatorade + sugar per 21 oz bottle. Two on the bike plus one in my jersey pocket will get me through a very long day.