If I have a 401k that has both Roth and Traditional contributions, then leave the job and want to roll it into another employer's 401k will that be possible? Will there be complications because of having both Roth and Traditional contributions?
Many 401(k) programs do allow people to roll money into the 401(k). The company running the program should be able to guide you through the process.
As long as the money doesn't change type, there should be no tax impact. Sometimes the money can be transferred directly, in other cases they have to send a check made out to the new program. You don't want the check made out to you, though sometimes they want to send the check to you and then you mail it to the new 401(k) administrator. The new company should be able to explain what they need.
One of by kids did this last year. The old program processed it as two transactions: Roth and non-Roth. The new company was able to work with the two checks.
If the new company doesn't allow employees to roll funds into the 401(k) you can either keep in the funds in the old program, or move them into a Roth and Traditional IRA. Sometimes if the balance is under $5,000 the old company will force you to rollover the funds.