IMO, if your shooting at f/4, you need more light. I commonly shoot at f/11 and above for products -and people for that matter- and to achieve this I use strobes and I crank them up hot. To get the same amount of light out of a constant source, you wouldn't be able to stay in the room long due to the heat. I also use a tilt shift lens for still life. I know not everyone has one of these but it gives a lot more control to, well, everything.
I have a friend that does nothing but product photography and giclee reproductions. He uses an old(er) 4x5 view camera of some sort -looks like something you would find at a flea market- and a digital scan-back... he gets like 160mp photos, its ridiculous. In his case, the constant polarized lights, which are extremely bright and HOT, work well. I don't want to know what he paid for them, not to mention the ventilation system he had installed to keep his shop cool when using them.
You don't have a spend a ton of money to get good results in this field. I still have my first set of strobes, Novatron v400 with four heads (real pos's by todays standards) and I use a PW to trigger them. I paid $100 for them at a pawn shop. I still use them for product photos because they are very bright and I can get f/11+ using them. I also use a lot of cheap fabric, cardboard, spray paint, tape of all sorts, paper, aluminum foil, you name it. The only thing that matters is the end product... nobody has to know what you used behind the scenes. To most, that $0.49 piece of paper looks like a $490 diffuser when reflected properly.
Your on the right track, keep it up. Sorry for the long winded response.
