I have heard that it's quite common for babies who start solids to wake more in the night - this can be due to them not taking enough milk in the day due to their tummies being too full from solids and also due to the food not "sitting well" on their tummies if you are offering solids later in the day.
I too am battling with the right quantity of solids to offer my 8 month old as he is waking twice per night and taking two full breastfeeds each time - I am actually going to cut back on his solids intake again (have done it once already).
4 months is pretty young to be offering solids, most don't start until 6 months so you are already ahead of the game from that point of view, so I don't think you need to increase as you are probably offering more than what most 6 month olds eat anyway. I would just stick with what you are offering and perhaps even experiment in cutting down a little to get more milk intake in the day.
Now, I haven't found the magic answer of for BF and solids - but last night our first NW wasn't until 12.55 (had been 9.30-10.30 previously) - I think the reason for this was that I cut down on the solids that I gave him at bedtime and as a result he took a great feed (I struggled so much before trying to get him to take decent feeds). Now, I have also decided to feed my solids not long after I BF him, so he's actually properly hungry for his next BF and will take a full feed.
I have definitely found that since I've been offering more solids that he's cut right down on the milk in the day and eats at night - as I said, I put this down to his tummy being too full in the day from milk. Although the quantity doesn't seem much to us, it is a lot for their tiny tummies.
Good luck!