Johnston's two losses in the last two games of the season meant that Llanrhian took the division three league title after weeks of being in second placed.
Division three had been a two-horse race for quite some time, with both Llanrhian and Johnston earning promotion.
Johnston finished over fifty points ahead of third placed St Ishmaels Seconds, the team which beat them on the last day of the season.
The bottom two teams which have been relegated from division three are Haverfordwest Seconds, who ended on 51 points for the season, and Llangwm Seconds.
Taking their spots in division three are Llanrhian Seconds and Hook Seconds, who finished top of division four.
Llanrhian Seconds were strongly in top spot with over 200 points, while Hook finished just one point ahead of third placed Whitland Seconds.
Kilgetty Seconds and Crymych are the two teams which finished at the other end of division four, as they prepare for life in division five.
They will be taking the place of Fishguard and Herbrandston Seconds, who earned promotion from division five this season.
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