Hi Blashnet, I firmly believe the 2 most important things when raising larvae are:
1. Localised feeding of freshly hatched brine shrimp.
2. 100% water changes daily before feeding.
The larvae need a concentration of bbs delivered locally. This can be best achieved in a small container. After my babies have finished eating they have big red bellys, but they still mooch out some the remaining bbs over the next few hours.
24 hours later they need a 100% change of water (another advantage to using small containers). Uneaten brine shrimp are like a magnet to bacteria and should not be left in the water for more than a day. I actually advise using a clean container each day - washing the dirty ones in a dilute bicarbonate of soda solution before rinsing and stacking for the next day.
This is my normal regimen and I find it very effective.
Regarding size, they're like kids, they grow at varying rates. The larvae from my leucistic mothers tend to develop slowly, despite receiving the very same treatment as the larvae from my golden albino mother. Having said that, there is usually at least one from a tub of 5 that out-grows his siblings quite quickly.
1.5cm at 4 weeks is not unusual, especially if they're from an axolotl's first spawning. I would now have them in pairs of equal size if I was you, in small containers. This gives them less competition for the food available and the next 4 weeks should show a considerable size increase. I would also be thinking of buying in some frozen bloodworm at around 5 or 6 weeks. You can tease them with individual bloodworm in a plastic pipette for a few days until they get the idea. (they don't need many at first, but they soon get a taste). I only give bloodworm as a supplement till they are about 8 weeks, then, when they start hunting down bloodworm for themselves you can put away the bbs hatchery.
