Can anyone give me advice? I'm due to start a full-time PHD in Oct this year in social work and hope to work 4 days per week as social worker as well, is it possible?

I'd say not a chance! For a start its unlikely that your supervisor would agree to it, and secondly the workload for a PhD is enormous, it easily fills 5 days a week! You'd be working every hour God sent and more. I'd strongly advise you to either do the PhD part time or drop to a max of 2-3 days a week working - I work 10 hours a week and I struggle big time to keep up, I work 7 days a week basically, you'd have to somehow get what would amount to 9 working days a week work in... burn out...
Having said that, I think some here do similar, but I'm not sure anyone does f/t work and f/t PhD, I don't see how you'd last the course

And if it's a funded PhD it's not just down to the supervisors: the funding body may put a very strict restriction on how much outside/other work the PhD student can do. Very strict. Nowhere near 4 whole days.

A full-time PhD is like a full-time job.

Yes, I agree with the others, a full-time PhD is full-time, and working 4 days a week would be extremely difficult. I worked 3 days a week for a lot of my PhD, and even that was too much and I ended up taking a year off to write it up. Work less - you'll get your thesis done quicker and won't burn out as much. A PhD is really hard, and you need to give yourself as much time and space as possible. Good luck!

The other alternative to full-time phd and part time work is of course part-time phd and and full time work. This is definitely 'doable' but equally tough, takes masses of dedication-particularly as phd will take 6 to 7 years to complete. I imagine social work is pretty demanding work so either option is going to be a tough call. Just completing a 25 thousand word Masters thesis part-time this year while working full time this year for me has been incredibly demanding and I'm still on the tail end of micro-edits which have eaten away my weekend this week and will chew into much of my holiday next week.

Both options are doable but I think you would really have to be superhuman to do a full-time doctorate and full time demanding and emotionally draining work-I would agree with the others, completing a doctorate is really hard work. I think sometimes we underestimate just what it entails.

Ii work full time, but as this is  in a school even though I don't get teachers' holidays I do get some extra weeks off (unpaid), I'm doing my PhD part time, and at times it is still difficult, for example I've had a few weeks off over the summer, but in that time I also wanted to do some decorating, and get the garden sorted, and neither of those things have really happened, but they are still there in the background doing the 'look at me!' bit when I'm trying to get on, which doesn't help. doing both elements full time I would say is nigh on impossible if you have anything else at all to do - like sleeping and eating for example (especially if you have to do the cooking too). I wouldn't even go there, and I doubt if your uni would let you do it anyway.

Thanks all, given me a lot to think about, I think it's just not knowing much about what extent of study is expected, my PHD is fully funded and I honestly think I will have to re think work, a bit worried about getting practice experience, but i'm sure I can get something more suitable for the study period. I really appreciate your input because I don't know many people who have completed PHDs.

Are you funded by work or one of the research councils? If its the latter then this question is null and void - they won't allow you to work more than 6-10 hours per week anyway :-) I think most people have no idea of the work involved, my family certainly don't lmao! Some weeks you'll have far more to get in than others and that's where work can be a massive problem. Some weeks I don't do a huge amount beyond reading, others I'm writing and burning huge quantities of midnight oil, working f/t would be a terrible pressure. As you're funded though your funders and your uni just won't allow it, you're basically paid a wage to work f/t on your PhD.

I have a scholarship to cover my fees only. I get nothing for living costs or research costs. The scholarship is for 3 years full-time study. If I switch to part-time I lose the scholarship so I have to stay a full-time student. Besides I want to finish in 3 years if possible. I have to work to pay bills and fund my research. I work 30+ hours per week. It is damn difficult but it is do-able. I have no other life apart from work and study which might not suit some people.