Q: My 4-year-old client has learned L with his tongue-tip down. Should I let him continue this pattern, or should I teach him to make a tip-up L?

In my opinion, you always want the tongue-tip to be elevated when it is supposed to be on T, D, N, and L.  You want your client to be developing oral movement patterns that will help him succeed all the way through to mature speech.  So you are teaching him things today with your eye on his whole life––not just what is good enough for today.  Whatever patterns you accept may lock in for a lifetime.

HOWEVER — It is normal to produce L with the tip down at his age so he may outgrow it.  But I would get that tongue-tip more active now.  His need for therapy for other phonemes suggests that he needs help with the overall process of speech movement learning.  You already are in the process of teaching him correct oral movements in many other ways and you might as well set the tongue-tip on the right course too.