Is my plant too small (in week 4) to get a decent yield next month?

First of all, if you're expecting to get a harvest in a month, you're going to be disappointed ... the plant would need to be in full flower right now to expect a harvest inside of four weeks. Second, don't believe the hype generated by seed sellers as to the time it takes to get a harvest - they want to sell seeds and advertising a fast harvest only serves that purpose.... most will tell you that their estimates are the BEST you can expect but that generally you'll need to be growing quite a bit longer. That being said, you've got a good looking plant here. I'm surprised she's not drinking more but she doesn't look like that's an issue... Once she goes into flower, she will start to stretch and she'll get a lot taller. Height is not an issue - I've grown plants that only got to be 21" and others that got close to 5-feet.. different strains, different conditions - it all makes a difference but genetics plays the biggest part here. Your lights seem to be much too far away - bring them down... no further away than 24" ... when she goes into flower, you're going to want them to sit about 18" above the canopy.... Good luck...