Not all 4-track Portastudios are alike,...
in fact, I think the Porta02 is actually a "Ministudio", and not actually a "Portastudio".
The Portastudio-type devices break out over a wide range, with the Porta02 being a stripped down unit, but still functional. It's still a better 4-track than nothing at all, but being an entry level device, most people outgrow the Porta02 very quickly, just like you,... having just started, and now needing a preamp or other device, just to give your signal that extra 'edge' it needs.
Right, live & learn, but don't fret about the Porta02, it's still a fairly good recorder to start on, given it's limitations. -Beats what I started on, which was a tape-to-tape kluge, with horrible sound quality.
For ~$150, the Porta02 fills a need for low end, entry level, no frills equipment. The 414mkII is ~$300, and gives you a kick upward in features and sound quality, and for ~$400, the 424mkIII is a feature packed, full function Portastudio, that's top of the line, and well worth the extra cost.
Anyway, don't fret about the Pora02, it's a good starter unit, and a preamp or other accessories should help you get better sound quality, over all.
As our other friend said, mic and mic placement is also very important.
Good luck, DA