I use something similar to this
for audio. With this I can still have the regular speakers plugged in while plugging one end into a capture card. Capture cards kill me with their delays, this way I can mute the capture software.
Of course I only occasionally record audio and never video. While I'm sure plenty would disagree with me, I record straight from the monitor for a reason, capture cards suck. Why would you need to post process it into 4:3 by the way? Will that card not accept 4:3 signals? If so, better to find an older capture card than post process it. The Amiga itself should throw out a 4:3 signal upon capture, so long as you're not playing an NTSC game in PAL mode.