It's either the basses are set up differently (or are completely different types of instruments with different tones as suggested) or you're overcompensating for the extra neck width by not putting as much weight behind your fingering. This will be even more noticable if you monkey-paw your 4, but can't on your 5 & 6. [personally, I derive a certain amount of satisfaction in monkey-pawing my fretless 6 for rock songs

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Your slap issue is a common one, so don't worry about it -- the solution is pretty simple. Slap harder. That and play a little lighter (and up your volume to compensate) when you're doing fingerstyle and you'll have a much less noticable volume difference.