Covering an Area in Sweetgrass

I'm very interested in information about sweetgrass. I'd like to cover an area 15' x 5' in sweetgrass. But first, I have a few questions about the scent of this plant:
Will the vanilla scent be quite strong when the plants are young?

Yes, the vanilla scent is noticeable from an early age.

Will the scent increase as the plants mature?
Is there a time of year that the scent is stronger?

There may be some seasonal variation or variation as the plants mature but we have not noticed much difference. We grow all of our sweetgrass in greenhouses so we may not notice as much seasonal variation as might occur in the garden.
Sweetgrass hydrosol is a strongly scented water that is produced during the steam distillation of sweetgrass leaves. It is typically made with leaves harvested during the warm summer months when both the volume of leaf material and the scent is highest.
Dry sweetgrass actually gets stronger with time.

What is the coverage I should expect from a 4" pot? I.e., how close should I plant these together?

Sweetgrass spreads by underground runners, so you can space plants further or closer apart depending on how quickly you want the area to fill in. Planting close together also helps to suppress weeds from getting established in the patch.
A sparse planting requiring a year or two to fill in and probably a fair amount of weeding to keep out invasive species would be 18-24 inches (45-60 cm) apart. Planting more densely at 8-12 inches (20-30 cm) apart is better.
Another option is to start from seeds. Periodically we have seeds available (available as of this writing). Sweetgrass seeds are strange in that they are mostly infertile and won't germinate, but the seeds that do germinate go on to spread by underground runners. A packet with 100-200 seeds will go on to produce 5-10 plants which will quickly spread to produce a thick patch. We do not recommend direct seeding in the garden; it is best to seed in pots or seed boxes and then transplant to the garden when the seedlings are strong enough to withstand transplanting.

Do you carry sweetgrass braids? I believe they are about 1' long. If so, what do they cost?
Do you have some on hand so I can sample the scent?

Sorry, we do not have sweetgrass braids for sale.