Position 1: Abadel (under 1000!)
So at first I thought Abadel's move: 1. Be7 was brilliant. the idea is 1... Bc1 2. Bd6+ Kxd6 3. e7! But it turns out this only draws after 1.... Bxe7 2. Kxe7 b4! (2... d4 is less good, since white can take and push the d pawn to attack the black king: 3. cxd4 c3 4. d5 c2 4. d6+ Kb7 6.d7 c1=Q 7. d8=Q Qg5+ 8. Ke8 Qh5+ 9. Kd7 Qd5+ 10. Ke7 Qg5+ and because of the advanced e pawn, black is only drawing)
So back to 2...b4! 3. cxb4 c3 4. Kf7 c2 5. e7 c1=Q 6. e8=Q and black is winning, at least according to Fritzy.
The best move is definitely 1. Bh6, simply intending 2. Bf8. Black gets nowhere with 1...d4 2. cxd4 c3 3. d5 c2 4. e7 Bxe7 5. Kxe7 c1=Q 6. d6+, since the bishop guards c1.
Position 2: (Rawn) Black had been defending this position for ages, and he made the psychologically classic mistake of 1... c5. The problem is that once the rooks are all traded, white can take on f6 and play h5, making a passed pawn. Nice teachable moment for resisting the urge to simplify! Instead, ....Rb1, intending ...Rh1, or ... f5, forcing white to fix the pawn structure, are both holding without too much trouble.
Position 3 At first I thought 1... h5 was the best, with the variation 2. Qxh5 Rh6 3. Qxh6 gxh6, but the truth is that 1.... Qxf1 is simple and obviously better.
Position 4 Najeebah's move 1. g3 is a horrible blunder, allowing 1... g4, making a passed h pawn.