Having similar issues at the minute myself, actually. Just got an ADF on Saturday but I'm not sure he's eaten anything and the betta is getting too much.
Sagat's been stalking and flaring at the frog, though. I saw him try to bite at the frog's leg when it was poking out of a plant but the frog is unharmed; still got the full webbing and moving normally. I only saw the frog twice briefly for all of yesterday, though and I was sitting next to the tank most of the day.
I do have something to try, though; got a spring-loaded 'tube within a tube' device, so you can put stuff in the interior tube and then when you press a button, it's released. The betta hasn't seen it before, so hopefully he'll keep his distance but it's finding the frog in the first place that's the current challenge. He surfaces now and then for air, but that's a split-second jumping out of the middle of an anubias bush or something then away into the undergrowth again...
Keep me posted and let me know what you find that works!
Re flaring and stalking: Hm. One of the frogs nips at the betta if he ever gets too close, so he keeps his distance and flies off if a frog comes toward him. He's just a chill betta, though, I've never seen him flare, stalk, or bite at anything in the tank.
Re finding them: I wonder if they're concerned because of the betta? My two frogs have been very active and out and about since the second day they were in the tank, they just hid the first. Sounds like you have good hiding places for them. I'd like to get a a couple more for their comfort, but first I need to be sure I can feed them! This 'one at a time' feeding thing doesn't lend itself to four frogs.
Re feeding: Last night I put my betta in a mock 'breeder box' aka tupperware balanced on the rim of the tank, took a small jar, filled it with defrosted frozen bloodworms and pellets, and put it on the substrate and encourage one of the frogs into it. He sat there. And sat there... I don't think he quite got the idea to eat the food, yargh. But it might be something to try for yours so that he's protected from the betta while he eats?
I'll respond to the pipette idea below-