The collar that hold the 4H cap in place should always remain tight, its only purpose is to hold the cap on. If you loosen it you will probably get a bad fan pattern.
I have a Q4 as well. It's way to strong for spraying canvas varnish! By a factor of 4, at least! Used without reducing the air flow a lot of varnish will simply bounce off the canvas and into the air. To get it down to an appropriate air flow I partially close the hose connector valve as Malcom suggested. I also put a garden-hose "Y" adapter on the output of the turbine. One branch does to the spray gun, the other goes to a 25foot "soaker hose" with porous walls that bleeds a lot of air. The resulting airflow is just right. The soaker hose also serves as a sound muffler. The Y connector and soaker can be bought at Lowes, Home Depot, gardening centers, etc. At least in the US HVLP hoses use the same threads as garden hoses.
You can get your paint flow in the ballpark by loading the spraygun with water, then adjust the needle to get about 200 ml a minute sprayed into a bottle. At that rate you can move the gun over the canvas at about 1 foot / 300mm per second with a deposition rate of about 8 ml per square foot per pass. 2 passes about ten or twenty minutes apart should do it unless the humidity is so high you shouldn't be spraying at all. I always weigh by gun before and after spraying to make sure I'm getting a consistent deposition rate and amount, that pays off in long term consistency. For water based sprays 1.1 grams = about 1 ml. For solvent varnishes, 0.75 grams = 1 ml. Stay away from solvent unless you are an expert and understand the serious risks.
If your gloss varnish is drying matte, you are probably holding the gun tip too far away. 8" / 200mm is optimum. Any further and the drops will get tacky on its way to the canvas. The varnish should look wet and glossy for at least several seconds after it hits the canvas, otherwise you will never get a gloss finish. If it hits the canvas in a tacky state you will always wind up with a hazy matte finish. Temps above 80F / 29C can be difficult due to excessively fast drying, in the summer I have to get up way too early for the 60F / 15C temperature that I feel gives the nicest finish. Find a dewpoint chart on the internet, buy a cheap temperature and humidity indicator, and never spray when the dewpoint is less than 10F below the actual temperature, since that risks hazing and other weird stuff.
Be sure to clean the gun very well and soon after each use. Water sprays gunk up spray guns really fast. I hate water sprays, total PITA!
edit: I used to do a lot of water varnishing. Timeless can be challenging even for those with some experience. A newby will probably get much better results with Glamour II, mainly because it has much better leveling properties, which is the ability to resist visual differences in the coating due to somewhat varying thicknesses. Learn how to dilute it with water using a weighing scale because it's so thick it's hard to measure with measuring cups etc.
