>>743640>>750681Technique is a huge part of not getting fatigued, this will be solved by just doing more kendo and paying attention to using the right muscles when striking, i.e grip strength to "flick" the shinai rather than using your upper arms.
As for actual stamina training, short distance running and interval training is great for legs and stomach. Run on the ball of your feet rather than you heel to increase calf strength/stamina. However don't get fivefingers as they will wreck your knees. I usually just run 2km as fast as I can twice a week. Basically you want to increase your recovery speed rather than your max running distance.
For forearms you can mix suburi with some dedicated forearm exercises. I would advise against this though as your form is likely to be pretty shit at the moment and practicing at home might enforce bad habits. Maybe do just some forearm exercises? Your sensei should be able to show you a couple. I could link a video, but I'll have to do some digging first.
But seriously, you've been doing it for four weeks, just give it some time and your body will adjust.