If you make it a habit that after every salat you repeat for 4 minutes estagfirullah after fajr; 4 minutes la ilahe ilallah after zuhr; 4 minutes el hamdulilah after asr; 4 minutes ja mukal-libel-kulubi.... after magrib; 4 minutes subhanallah after Isha.You have absolutely no intention of competing the sunnah. Would that be considered a bid'ah that competes with the sunnah and thus impermissible and sinful???
Simply speaking bid'ah is an innovation in the religion. This means an act of worship which have not been known or practiced by Muslims at the time of the Prophet.
What you describe is a certain dhikr or as sufi's (and maybe others) may say it is a kind of wadifa. Which is a set of dhikrs one intents to say continuously at certain times.
The dhikrs you mentioned by themselves are certainly not bid'ah, but known from the sunnah the practice of the prophet himself.
And as long as you don't think what you've done is a good worship which you invented and which should be performed by others (in the same way) it still isn't bida'h. You should do it for yourself and don't invite others to do it similarly to avoid making it a "sunnah". As early scholars used to say: "A person who think that he introduced a worship or good deed that was not done by the earliest generations of Muslims, such a person is equal to one who said Muhammad didn't reveal the complete religion".
Finally have in mind that performing the dhikrs which the prophet recommended after the prayers is still the best thing to do.