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That kinda depends on pretty much everything. If t were me, I'd go hot and fast to in IT of around 140˚. For a 4 lb. leg, that should only take a couple hours at most. Low and slow maybe 5 or 6 hours, depending on how you want the meat. Bone in or boneless will affect times as well.
I want to say it took me 4 hours to get to 145 last time I smoked a leg of lamb that was about the same size but boneless. I was keeping the smoker around 235.
I did a rack of lamb yesterday.The finished product is my profile pic. I used a digital meat thermonitor & let it smoke on the grill at around 300-350 away from the coals for about 1 hour. the only seasoning I used was fresh ground sea salt,fresh ground black pepper, & fresh minced rosemary from my herb garden& soaked cherry wood chips on the coals. It was my 1st attempt at lamp & I was happy how it turned out!
I can only do it in metric,but I went 3x hours at 90c & 1 hour at 100c in my Sardinian lamb post.I had a probe as well. That gave me medium rare. Boneless but 2.4kg bone in.