What's the proper technique for starting uphill?
Based on the Sheldon Brown article here's my approach. I find that putting the pedal 45 degrees forward of top dead centre is too far for maximum starting impulse - I go more vertical than that. I also start with my strong foot resting on the pedal, but that's partly because that means I lean the bike away from traffic and towards my single pannier when stopped. 

Starting out on a hill I'm forward of the saddle. I push forward with the foot on the ground as I step up on the pedal, which puts the saddle under me. 

But this depends on being in a suitable gear. And as you don't have a choice in this respect that sets an upper limit for your starting gradient. Unless your gearing is too high for the riding you do (and don't rule this out) you should be ok starting out on 4%. I have about a 6% start regularly, in a bike lane, and would be in something like 38x13 on 700C wheels - so a similar ratio to many single speed commuter setups I've read about. A little lower would be better, a little higher is possible. 

One tip if it's a traffic light start in a bike lane: start as  wide as you can in the lane and get going as soon as sensible (this is both a personal and a local matter). That way any wobble out of the bike lane is done while you've got some space around you - but you've got room to wobble towards the kerb.