If +1 con bracers and 18 ([color=red]4[/color]) con that makes you have a con of 19 ([color=red]4[/color]) then your mod is still 4 and doesn't change your hp
con 18+1 pair of bracers = 19 con your mod only goes up on even numbers
@ con of 20 (5) then you get one more hp for each lvl you have