If a #6 line is what you want, or feel is the correct choice I would stick with it.
I reeled in a 22 inch Salmon the other day with a Fli 6wt and found it to be just fine. My wife has a Fli 5wt which is also a nice casting rod.
In my opinion and with my casting skill I feel a 4 wt rod is a little light for most of the heavier streamer flies. For the money a FLI is a good bet.
What type or size fish are you catching ? What type & size flys are you using.?