About 6.5ish PH going in, but the runoff is down around a 4.5.
Runoff ph isn't very accurate, but it's accurate enough to say that's a problem. It's better to measure runoff ph and ppm, the trend of each will warn of salt buildup (overfeeding and/or insufficient runoff) before it turns into nutrient lockout. Usually soil acidity results from overfeeding, but 4-weeks old sounds a little young.
How do your plants look? For me, mine start showing lockout when runoff ppm reaches 2500 and ph drops below 5.0-5.2. These two numbers seem to track each other for me. After experiencing nute lockout a couple times I found the appropriate amount of food and runoff to avoid buildup, and I stopped watching runoff except for occasional checking.
Your acidity may not be due to salt buildup. It seems like FF soils have problems, but it could be that new growers use them more (and the problems are due to other things).
Are you letting the soil dry completely before watering? The soil ph rises as it dries. It should be almost as light as paper (but before the leaves start drooping, although it's not bad to let them droop once or twice to learn how dry is too dry, learning how light the container feels).
How much runoff do you get with each feeding? I think >20% of the container size is always good. (If you were doing true organic soil, maybe you wouldn't want that much.).
It won't hurt to do a modest flush the next time you feed. Do half-strength of what you normally feed and mix enough to get 100% runoff. But, only do this after letting the sol completely dry.
You can feed/water at 7.0-7.5 to help pull acidic soil higher.
Adding 1tbsp/gal fine/powdered dolomite to the soil before planting seems like a common practice. It helps hold the ph stable. In cases like this people will cultivate it into the top soil with a fork and water it in. But, it takes 10-14 days to have an effect and probably doesn't have an even effect.
If the plants don't show signs of a problem I wouldn't do anything drastic yet. Maybe water higher ph, more runoff, start watching the runoff ppm too.
If you want to see why runoff ph isn't accurate google for NCSU Pour Through Method. It has to do with how long the water equalizes to the soil ph and how much new water displaces (and potentially dilutes) it into runoff. But, the risk should only be that the runoff ph will be higher than it would be if following that process.
