Defensive errors and missed chances cost the Linnets dear on Saturday.
The Tudors won their first home league game since August as Lynn, who had taken the lead twice, stay top, with the chasing pack mainly closing the gap.
Linnets manager Keith Webb said "It is true when they say football is won in both penalty boxes.
"We set off in the right way, and were deservedly a goal up.
"We conceded from a corner, which I'm annoyed about. The second ball we didn't defend well enough – which was the story of the game.
"At 2-1 up at half-time I knew we were in for a tough second half against the wind and the slope.
"Had we not conceded from set-pieces I couldn't see them opening us up.
"We had a great chance but put the ball over. From their goal-kick we conceded a corner, which they scored from – all in 45 seconds. That got their tails up.
"They scored from another set-piece, but we had an opportunity to equalise, but didn't even work their keeper.
"The fourth goal was just a comedy of errors. We just didn't capitalise on our opportunities.
"The set-piece goals conceded are a concern and what we talked about at half-time.
"It's not as if people out jumped us, but we weren't alert to the second ball and knock-downs.
"That happened three times in the game. Something's wrong, and we'll look at that in training this week."
The Walks outfit took the lead on a glue-pot pitch with the first real chance of the game on 13 minutes. Good play down the left from the two Bens, Chapman and Sedgemore, set up Joe Francis, who was fouled in a clumsy challenge by Chris Herron.
Danny Bloomfield sent Ian Brown the wrong way, firing to the keeper's right, from the penalty spot.
Bloomfield's effort was blocked by Brown, while Greg Crane missed with a header at a corner and Bloomfield could not convert a Chapman cut-back in Lynn's best spell before the half-hour mark.
Hemel levelled on 36 minutes when a swinging corner by Herron produced a shot blocked by Lynn keeper Scott Howie, and Adam Martin rifled home unopposed from six yards.
Martin and Herron missed the target before on 45 minutes Lynn broke away down the left and Jack Defty turned provider for Bloomfield to glance in his 17th goal of the season, to lead at the break.
The turning point came on 51 minutes when Lynn wasted a four-on-two situation. Steve Melton picked out Defty, who opted to curl the ball, which went over.
Within a minute Hemel went ahead at a corner.
The ball was cleared to Herron, who fired in the goal of the day with a 25-yard left-foot dipping shot.
A defensive mix-up saw keeper Brown slice his clearance under pressure from Defty, and Bloomfield was inches over with a 35-yard lob.
A long delay with a quarter of an hour to go was caused by treatment to Herron, who collapsed with exhaustion following a virus, and who later made a full recovery.
As a rollercoaster game entered time added on, a high corner was not dealt with by the Lynn defence, who failed to clear and the home captain Paul Edgeworth forced the loose ball in from two yards into an unguarded net after 90 minutes.
Lynn went up the other end and sub John Turner spurned a great chance to equalise by heading over from six yards.
But in the sixth minute of stoppage time a horrendous mix-up between Mark Camm and Howie let in sub Danny Nicholls to complete the win by scoring in an empty net.
Hemel: Brown, Herron (Hunt 76), Coulson, Cavill, Bowden-Haase, Martin, Gearing (Burgess 75), Cracknell, Sozzo, Roberts, Edgeworth. Subs not used: Yeboah, Mannion. Booked: Gearing.
Lynn: Howie, West, Chapman, Camm, Crane, Melton, Buhlemann, Sedgemore, J. Defty (Turner 75), Bloomfield (Nolan 85), Francis (Fisk 59). Not used: C. Defty, Murray.
Ref: Alf Field (Chesham). MoM: Ben Chapman, outstanding commitment.