Dànshì vs kěshì vs rán'ér vs bùguò
Other than the formality of the tone of your expression, the differences in these words are very small.
但是-Nevertheless, but
可是 -but
然而 -but (the most formal uage of 'but or however', it is also 'yet')
不过-but (the most informal usage of 'but or however' in Mandarin Chinese)
They are used interchangeably at varying degrees.