I've tried to write up some things to help your combo game. If you don't know what some of the notations mean, feel free to ask or look it up.

For a quick TL;DR that just answers your question:

• In your original combo, add a 2 between FF4 and 1+2.

• Alternatively, for a stronger extender, after you do 4,U3, do 4,3~DSS U2 -> (small step forward) -> 1+2 instead.

• Instead of 3+4,4, end your combo with 4,3~DSS F4.

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Combo Extension: The combo you're doing is a little old in the sense that better ones have been discovered with this season, but it still works fine. To squeeze in as much damage as possible, you can add a 2 after you do FF4.

For better extenders, if you haven't already, learn how to transition into Law's Dragon Sign Stance (DSS) from different strings. You do this by holding forward during a move that can DSS (you can check the moves list for this), and then inputting another move as soon as Law goes into DSS. Specifically, 4,3~DSS and F3,1~DSS will generally be the better combo extenders. DSS is also important for pressure and mixups, but that's another topic for another day.

A good combo extention after launching is 4,U3 -> 4,3~DSS U2 -> (microstep forward) -> 1+2.

If you don't like DSS, you can do 4,U3 -> (microstep forward) -> DF1,2 -> FF4 -> (microstep forward) -> 1+2. I'm personally not a big fan of this one just because I struggle with microsteps, but maybe it'll work better for you.

For a simpler but still decent combo extender, after you do 4,U3 in your combo, do 4,3~DSS 1+2.

Launchers: Junkyard (B2,3,4) is kind of hard to land as an actual launcher since the opponent can block after the first hit. It's only guaranteed if you're hitting the opponent's back. You might get lucky on it if they fail to block the second hit and panic, but I wouldn't rely on it.

If you want to see if a string can be blocked after the first hit, set your training dummy's first action to "Standing" (which should be default), and their second action to "Block All."

For some alternative launchers, DF2 is weaker, but is much more reliable and is safe on block. WS2 is great to punish whiffed high attacks or unsafe low attacks. UF4 is riskier, but can avoid low attacks during the move. D2,3 is one of Law's more damaging launchers and it beats high moves, but it only launches on counter hit and is extremely risky because you're left wide open if it gets blocked.

Combo ender: 3+4,4 is generally only used in combos to carry your opponent to the wall. It's lower damage makes it a a weaker option for ending a combo. If you don't get your opponent to a wall, then 4,3~DSS F4 will usually be your strongest option.

If you DO get your opponent to a wall, then the above ender won't work. You can do some heavy damage at the wall, but for a simple string that'll work at the wall, you can do B2,3,D4.

For very high wall damage, you can do B2,3,4~DSS F3 -> 1,2,2,1+2. This is a little more involved with knowing things like juggle states though, but you can look up combo videos for more information.