Just what I noticed on a quick playthrough, take with a grain of salt.
i think you're right,
should be at D10.
I don't think you can make this cut. What if black plays B3?
probably better to just play r14 or q14
I think maybe R8. Safer, at the cost of a little endgame.
It may be too ambitious, but that's because black is thick, not because the players are 4 kyu 
Any attack would have to start with a footsweep around s14 I think.
Anyway I'd just connect at F12. Black still has a lot of problems.
black's all connected now, white's attack failed before it got started 
feels small. J17? M3? P3? There's some risk of a cut at F13 also.
pretty sure white can block at S6, the cut doesn't work.:w101: Maybe let them go and play O4?
:w109: What about L3 instead?
:w123: can't play a, black can capture with O6.
:w133: You have a clever play here: R4 is sente, because it threatens a ko. Then you don't have to respond to this move.
You may be interested in some of the games that
is close to an overplay. There is already a cutting point at D13 and now you create another one at D10. It looks like B cannot cut yet, so you can play this way for a while, but sometime soon you will need to go back and add a defensive move. If you do not want that obligation, it would be good to continue simply with D10. Now the cut at D13 is no longer possible (please verify), so if B jumps out, W can take sente. 
is too thin. This should be C8, to help defend the cutting points W has left behind. After B cuts, W may be able to handle the fight, but it is painful to see the W wall crumble.
is bad but praiseworthy -- bad because the B stone is already captured, praiseworthy because this time you did go back to fix your perceived weakness. Sente is very valuable at this stage in the game, so it is worth some reading to see if you can omit this defensive move. Maybe you can assure the capture in a more positive way, say by pushing at F4, which is probably sente. 
feels like the wrong direction to me. The right side is more open than the top side, so it seems more appropriate for W to extend from the corner than to pincer. In addition, though it may not look like it, the pincer is potentially very dangerous. If B simply jumps out to O15, W will have three weak groups in this area. After W defends the corner, it is a very real possibility that B can drive the M17 stone towards the E13 group and engineer a double attack.
blocked the wrong side, as you realized a few moves later. The general rule is to block to block the wider side, in order to stake out the largest area possible. But since W lacks an extension along the right side, B gets an easy invasion, with room to make a base in either direction. In addition, the cut at N16 is left behind as a serious weakness, given the strong B position to the left.
could certainly capture the two B cutting stones cleanly, as you remarked. This would be another praiseworthy but possibly inefficient move. You really need to read to the best of your ability whether the B stones can escape and cause trouble. Then trust your reading and play accordingly. Even better, count the game and see whether a safe capture gives you enough territory to win.
to 
is an interesting trade. It seems a shame to give up the attack on the weak B stones, but I think W gets adequate compensation for this to be playable. Or was this a misread, not noticing that the W cutting stones get captured?
is a nice invasion, but it leads to a very complicated fight. Also, I do not think B can make a very large territory around there even with another move. I would prefer to play first in the upper right. The W stones there are starting to look weak, and keeping them strong is worth a lot. S14 is a huge point, undercutting the B base. R13 would be even better if it worked, say after descending to S16 to threaten the corner. W could also just play something really simple like L15, taking care of weaknesses first, letting B worry how to defend.
should block at S6, saving a full move, because B has to live in gote in the corner. Alternatively, after playing the game move, W should tenuki, as S6 is just a small endgame move once the corner is alive.
, the fight gets too complicated for me.