I'm not that bad at playing guitar and sometimes I come up with useable ideas, but most of the time as soon as I pick up the guitar and have nothing to lock on I start improvising melodies/licks/riffs but they always hardly in 4/4 time or with a even structure (maybe the phrase will repeat after 3 bars).That happens expecially if stuff I come up with has syncopation. I dont have problems with metronome or drum machine emphasising the beat one and I use them a lot but I'd like to have more an "internal clock". So I was wondering what I could pratice to improve my songwriting in being more "correct". Hope someone could help.
Let me start by saying there's nothing wrong with syncopated rhythms or alternating time signatures. Chopin and Liszt excelled in this in the classical realm; for more modern examples, I recommend Dream Theater who are absolutely phaenomenal at this technique.
Having said that, if you'd like to practice writing riffs for 4/4 time signature, your best bet would be to set up a drum machine with a nice 4/4 rhythm (with a nice strong accent on the 1st beat) and try to play along with it.
Alternatively, you could try things on paper first; having to confine yourself to a strict structure on paper will help you create more meaningful melodies, where the right progression / accent in the music, falls on the right beat in your 4/4 score. This allows you to also experiment with syncopated rhythms alongside, while maintaining some sort of discipline in terms of trying to visibly pair accented parts of your composition enough with the 1st beat of a bar at the same time, so that you don't stray off the sensation of a 4/4 beat too much.
Another insight is the fact that music is almost like words in a sentence, in that it has natural parts with accents, and natural parts without. Realise the similarity to speech, and attempt to spread the "words" (i.e. your melody) such that the accents in your sentence fall on the accented 1st beat of a bar most of the times.
Finally, just listen to some pop music; most of it is 4/4 anyway. If anything, most people have the opposite problem, i.e. how to compose music that isn't 4/4 !
If you want to compose in 4/4, get a sequencer, set up a 4/4 drum loop - maybe just a basic rock or swing pattern - and work to it. But there's nothing 'incorrect' about non-4/4 times, or 3-bar phrases.