I do agree that it's good idea to have a blocker hand when we consider a 4bet bluff but it's not always the case. The first thing to consider should be your fold equity preflop and postflop as well (bare this in mind, it's very important)He might defend often vs 3 or 4bet but play fit or fold postflop. You said that you perceive the 3betor as weak but weak could be relative. Even hands like JT,QT,KT has 45 (50 if they are suited) equity against yours.
If V calls often you'll have some trouble postflop. You wont like a lot of flops and even if you do it will be hard to find value. Hands like 77-99 as i like to say are "spoiled" hands (if this is the correct expression). In order to make 3 streets of profit with them (without hitting a set) you need certain conditions: like dynamics and board. Otherwise is very hard if not impossible.
All in all first thing to consider before bluffing is your "overall fold equity". For example If you deduce that some of your opponents is playing scary money or become way too tight when wins a big pot then you could profitably tore him appart ouf of bluffs and widen your range significantly. In a spots like this it's not mandatory to have blocker hands
This will keep your 3bet or 4bet very high, your image will be more "monkeyish" and when you have premium hand it will be more likely to be paid off