Use your gang upgrades
Upgrading your gang not only gives you stronger NPC allies and more transport options, but unlocking the Little Rooks early gives you a team of child pickpockets to bring you money and other assets.
Keep your gear up to date
Plenty of missions reward you with new gear so it's worth regularly checking to see if you collected any better stuff once in a while. You'll also collect crafting ingredients all over the place so make sure you're not sitting on something useful you haven't made yet.
Get all Sherlock on the streets
On PS4 you get extra Dreadful Crimes missions which add a whole other gameplay strand as you employ your Eagle Vision to scout out clues and dispense justice.
Check out the Thames
London's famous river is great for two reasons. Firstly, it's an amazing location and crossing it by leaping from ship to ship is always a thrill. Secondly, you'll find a few enemies supplies you can sabotage to get some extra XP.
Pay attention to the side missions
Side missions let do things like enter fight clubs, races or hijack enemy cargo. Not only will it boost cash, supplies and XP but it can unlock gear and other useful options. Make sure you take a look.
I've mentioned crafting once already but don't neglect it. Some things can only made be rather than bought and it's a good way to get better weapons. Keep a look out for crates in the world and invest in the lock picking skills to open special ones. You can also pick a skill that increases your chances of finding rarer materials.
Listen for music
As with every Assassin's Creed there's a ton of collectables. Although the most interesting are the music boxes. Collect all of these and you can unlock the secret in a hidden vault, so keep an ear out for music as you explore.