It is extremely tough to say what is the right shot without a proper video, because it really "depends". But I can give some pointers to start a thinking process based on some "wild guess".
From the below two quotes, it seems like you really enjoy the "amazing shot" feel of that clean down the line winner. If your opponents are of similar level, you SHOULD NOT get chance to hit "A LOT OF" down the winners, without sacrificing a lot of errors and easy net point finishes (the big opening between you the baseline returner and your net partner, gets exploited so easily). It is extremely easy to forget these silly mistakes, so video a full match and then rewind and analyze.
In a match it is important to keep the opponent net-guy guessing, and to keep him honest about his position and movement. The role of down the line winners is ONLY that. Anything more than that, there are some level differences in play or weak serves (for level) in play.
off a 2nd serve I can usually hit it pretty well.
Since then I've hit a few winners down the alley in other matches
Here is the role of play comes into picture. So being a 4.0 your main task in hand, is to get your higher level partner involved, and to avoid getting your opponent 5.0 player involved. Hitting winners is NOT your main task (but nothing against it, if it is obvious). Being the lower level player YOU ARE the target. Keeping you in play, and making you "think" you can hit winners (and there by avoiding your partner getting involved), is perfect for opponents.
If there was ANY way to avoid the 5.0 net player, that is the percentage play for you. You probably should have asked your partner, what exactly was the "percentage play".
Was lobbing over cross court to get the 4.0 hit the next shot a good percentage option? If yes, I can see that being very advantageous to your team.
Was keeping it low by a short slice and making the 5.0 hit a difficult volley only to get finished by your partner an option?
How far from the middle line was the opponent 5.0 net player?
How many times had he poached beyond the middle line and hit a clean volley at the foot of your net partner?
I was playing with a 5.0 vs another 5.0 and 4.0 where the net man was the 5.0 and very good
In general, 5.0 does see the game a little bit beyond what a 4.0 sees. (Just like you can see the percentage of points in play better than a 3.0).