The other answers have explained where the misunderstanding was, but on guitar (or bass) an alternating 1-5-1-5 is so simple, it's not even necessary to know what the 5 note is. Others will doubtless say the opposite, but here it is anyway. If the 1 is on the bottom string ( and you will know what 1 is, even if you don't know what 5 is) the 5 is on the 5th string, 2 frets higher. If 1 is on the 5th string, 5 has two homes - same fret 6th string, or two frets up on the 4th string.
It'll be there always, unless it's a dim or aug 5, which won't happen very often. Only kidding about the importance of knowing notes, but in this case, on guitar or bass, I doubt if anyone actually thinks about the name of the 5th. Just play!
Check out a previous question - 'Why intervals are not named after distance'.
You asked about if the bass note is D open 4th. Well, the 5th of it is the open A string - same 'fret', one string lower. There is another D at 5th fret 5ths string, and yet another at 10th fret, 6th string. Or, next string higher (in pitch) 2 frets up, gives 3rd string 2nd fret, like I said.
The bottom string is what we call low E - the fattest string.