Jim currently needs a further 59 runs to become the Welsh county's first batsman to a thousand first-class runs for the summer - and the first since 2011 to reach this milestone - whilst the forthcoming LV=County Championship game against Leicestershire at Swansea will also be his 100th appearance in first-class cricket.
Glamorgan Cricket's Chief Executive, Alan Hamer said:
"We are delighted to have retained Jim's services for the next four years. He is a key member of our playing squad.
Today's announcement re-affirms the Club's determination to become a successful first-class county. We remain committed to retaining our best players and to challenge strongly in all forms of the game."
Jim Allenby added:
"I am extremely happy to commit the next four years of my playing career to Glamorgan and look forward to working with the playing squad and coaching staff to bring success back to this Club."
Looking back on the season so far as Jim prepares to play his 100th appearance in first-class cricket, he said,
"Batting-wise it's been great for me this year, it has been my best season. My bowling hasn't quite been up there but I think that has just been a reflection of the dry sun we have had and the fact that you look around the country and there have been a lot more guys scoring big runs and less bowlers getting the wickets.
"Last year 2 or 3 guys scored a thousand runs; this year we should have at least 2 guys just in our side or hopefully 3 doing it.
"I think I have batted well, certainly my mind-set has been a lot better, my frame of mind has been as good as it's been in my career so that's reflected in my batting. "
Looking ahead to the next four years at Glamorgan Jim added,
"A four year contract means that this time next year I'm not sitting here having to go through it all again. It means I can help the club to build for a sustainable period not a quick fix of a year or two trying to patch over things, it really means a commitment to get a structure and a process in place, i.e. long term, well after I've finished, to build a successful club.
"The idea is it's also security for me and the club was happy with it which is great. It enables me and the club to set long term strategy plans as to how we are going to be a good side for a period not just for a month or two here and there because I think we have seen enough of that, I've seen enough of that and the supporters have seen enough of that!
"We have got to look more to build an actual production line of players and a style of playing that is sustainable for many years.
"I think it is a statement by the Club and from some very nice messages I've had it's what a lot of people have been thinking. It's something Alan's made no secret about, he's wanting to keep his better players I'm considered one of them so it is absolutely a statement of intent.
"It's not come lightly, four years- that's a big commitment by the club financially and time-wise - I think it's showing that they mean business now. We are not here just to play a bit of decent cricket every now and then, we actually want to have a good long period of success.
"I don't really know what else the Club could do at the moment I think the Club is doing absolutely everything they can to be the best possible side we can be.
"I know I'm not Welsh by birth, but having been here for 5 years and now another 4 at least I hope to be considered Welsh-ish ! I want to be playing with guys who are playing for their home clubs because I know that no matter who you are it means a lot to you to do that.
"I'm going to do everything I can to help not just the current first team but also to help the guys coming into the side because I think I'm good at that. And I want to make the guys that come in comfortable and see them improve so by the end of that 4 years and hopefully longer if I'm still good enough, we want to see the young, and the not so young, Welsh players continue to develop and get better like the rest of us who are also improving with age."