Adding sugar will thin the beer. I'd personally look to add DME, using atleast 1 for 1 with sugar. Sugar also 'drys' the beer, making it less sweet.
8 grav points change is about 1% abv, you are after 1.4. so about 11 more points of gravity (ie going from say 1.050 to say 1061). But this is more complicated. Sugar ferments 100%, DME ferments at the typical attnuation of the yeast, about 75% -it varries, but 75% is a good general guess. So thus for 11 points of Delta, you need like 15 to 16points of DME.
on a 5 gallon recipe, 1lb of DME adds about 9points, so in our case you need about 1.5lb of DME - well that would be 13 points.
Another options woudl be to use a mix of DME and sugar, based on my prior paragraph, you could add 1lb of DME for 9 points - but only 7 of change, needing 4 more points. 1/2lb of Sugar woudl get you about 4 to 5 more points. Total OG change +13 to 14, total FG change +2 or 3. ABV about + 1.4%
In general no more than 20% of the fermentables shoudl come from adjuncts. 80% or more from malts.