guitars are (for the most part) intended to sit in the midrange... so finding a good level for those first is key. if you use a lot of gain on your rhythm guitars, try pulling it back a little bit in effort to reduce some higher frequencies masking with other instruments.
don't be afraid to lowpass the guitars aggressively. the natural frequency response of popular cab speakers is around 7k so you can cut all the way down to about 7.5/8k without losing detail. guitars also benefit from limiting over basic compression because a limiter reacts a lot faster. also, to bring out more clarity and/or detail in the higher frequencies in the guitars, it would help to introduce a harmonic exciter in the mid to high mid frequency range. this would be done (of course) after applying whatever necessary additive/subtractive equalization. for instance; it's pretty common that a high gain fizz lives around 4khz. it is also common that a certain muddiness lives around the 400-800 range (depending on many different factors).
it almost always helps to employ a high pass filter to guitars. anywhere around 100hz would allow the bass to blend better with the lower-mid range of the guitars.
drums are difficult to mix (for obvious reasons). you must treat all the instruments as if they were pieces to a puzzle. some frequencies actually sing together though so it is important not to completely subdue one or the other as the is a musical piece, not a appendectomy.
bass guitar can be cumbersome because, let's face this with determinism... we are all trying to achieve a bass tone we have heard in the past. or some chronicle bass tone that has inspired the current attempt our bass mixing experience. ***philosophically speaking (i guess). it's simple: bass needs to be a lot less than you are currently mixing it. best bass is achieved by almost completely lowering the dynamic range. so; multi band compression, limiting, more compression, more limiting, etc. etc. there are also tons of frequency enhancing techniques out there, like using maxxbass, automation (fader riding) and a ton of sidechaining techniques as well. one very important thing about bass with regard to drums, you will almost always get a perfect fit/blend of kick and bass if you track the bass guitar with the cab placed directly where the kick drum was. the beatles did it, zeppelin did it... and so on. it works!
tbh, i wouldn't boost 4k into anything accept the master... and even that is a stretch. 4k sits really harsh in a mix and is sometimes almost unbearable to listen to at high levels. there are certainly eq's out there that treat any frequency with linear phase distortion and when boosted at any frequency can make any instrument sound like butter cream ...but let's face it, if you had one of those 5-10 thousand dollar eq's you wouldn't be asking most of these questions.
make sure you leave plenty of headroom for your mix to sit nice and quiet on the master bus. if you are mixing directly to a master chain it is important that you pay attention to the gain stage of each plugin, instrument, etc etc. nowadays, things change quite a bit during the mastering stages therefore create tons of noticeable disparities in the piece that may seem like small inconsistencies (at first) but then play a huge role of how bad the song can end up sounding (if that makes any sense).
the fact that you can hear the difference between your mixes and other commercial releases is a good sign that you have what it takes (for the most part) to change your workflow and critical listening methods in an effort to improve your mixes.
i don't really know if i answered any of your questions with exactly what you wanted to hear ... but i sincerely hope any of this helps you.
live long and prosper!
