What major differences for similar settings?
SC4 does allow more extreme settings. Also a soft knee, however that helps.
Probably the biggest difference: SC4 does not use lookahead.
Aha. That’s a big deal. So it simulates a real time compressor, with the limitations of such.
Whether that is a “limitation” depends on whether you want that behaviour or not 
Some good effects can be achieved by using the SC4 followed by a limiter (which is a common configuration when using hardware devices).
Reverse the track , apply SC4 , then reverse it back : now you have lookahead compression ( I think 
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If you want lookahead, probably better to use a compressor that has lookahead 
Personally I find using a compressor well without real time adjustments is difficult enough without Preview playing reversed audio.
Do you have an opinion for a good limiter effect for speech?
I understan d limiting is compresson with some lookahead, high threshold, short attack, and high ratio… Is te Hard Limiter really an infinity to one clipper?
Should I limit before or after other compresson?
if you are only wanting to apply a modest amount of limiting, I like this one: Missing features - Audacity Support
For more aggressive limiting, I’d go for: Peak Limiter
(I’ve been considering changing the name of “brickwall.ny” because there is some confusion between this and a “clipping” effect. “brickwall.ny” is a “limiter” not a “clipping” effect.
If you mean the LADSPA effect, then yes it is a clipping effect, but also provides a degree of “reconstruction” (adding back a scaled version of the peaks) so as to moderate the hardness. There are also a couple of “soft clipping” effects here:
Generally I would apply compression first, particularly when using a compressor that does not use lookahead (such as SC4).
If the uncompressed audio has occasional high transients, then you may want to use a limiter to moderate those first (more likely with music than voice recording).
For performing “loudness maximizing” it can be useful to apply limiting both before and after. The first pass to handle transients, and the second pass to squeeze out just a bit more loudness after using a lookahead compressor.