Yep. Our OT ordered one through wheelchair services for our DS2 who is 4 (almost 5) years. It has made such a difference for us!!
DS2 is unable to cope with walking other than very short distances. Because his communication skills are practically nonexistent when he is upset, we cannot seem to get to the cause of this. We THINK it's due to overstimulation from everything around him (cars, weather, walking, pavement, other people, noise, and so on), so he just very quickly falls apart - he either sits and refuses to go any further or he literally lays down and "shuts down" and will not communicate or even respond in any way. As he is falling apart, he become nonverbal, clinging, upset, clumsy, distracted.... and it goes downhill very rapidly, within minutes, so it's not always predictable as to when this will happen. We have to keep him highly focused on ONE particular thing - usually some type of verbal game that involves constant reassurance and chatter to him as well as numbers and letters as they are an obsession with him - in order to get him to walk for any distance, and even that is sporadic. He was reaching the point where he was refusing to try to walk any distances outside the house as he couldn't cope and I believe was associating it with upset.
Anyway... we had a long discussion with the OT about this. I specifically requested the Maclaren Major, pointing out to her that he was most comfortable and secure at this age in a pushchair, and that it was something that was familiar to him (as both he and his younger brother have used one), rather than a wheelchair. He is getting too tall/heavy at 4yo for a buggy board and too large for a regular pushchair. And that due to this issue, we were not able to take him to activities and opportunities that other children his age would be able to attend (days out involving walking about such as entertainment venues, longer walks in nicer weather in summer), plus it was such a problem that he was arriving at school ridiculously stressed from the walk to school in the morning which was not helpful for him to learn in school. It also is sometimes a necessity due to his poor impulse control, so that we can keep him safe when we're in a busy or crowded environment.
The OT is required to make sure that this is not a situation where you are looking to make the child reliant on the pushchair or pop him into it all the time so that they get no exercise, as obviously that's unhealthy.
I told OT that this (ironically) would actually allow us to encourage DS2 to walk MORE, as we could bring along the pushchair as a backup, so he knows he has the option to sit in the pushchair if he gets overwhelmed. We could not take him on walks at all, as we were concerned that halfway there or back he would refuse to walk back or get upset and refuse to go. Then we ended up carrying him wherever we were going, resulting in DH having a sore back and myself having a sore back and shoulder. So having the Maclaren along ("giving mummy's handbag a lift" as DS2 says 
) means that we breakdown the walk into segments, and give DS2 encouragemend and little rewards for each part that he walks.
I do think that it stood in our favour, that we explained quite readily that the pushchair would be used as a tool to encourage more walking, rather than allowing him to be become dependent on it. We try to do a lot more walking now that we are not worried about having to carry him the rest of the way. And he seems to cope with the incoming outside noises better, knowing that he can go into the pushchair and shut everything out, give himself a few minutes to pull himself together, then get out and try walking again.
The wheelchair services delivered it pretty quickly. 2-3 wks, IIRC. OT told me later that it was fairly uncommon and rarely heard of that they ordered Maclaren Majors for 4yo, but that given our reasoning about being there as a backup so he can actually TRY to walk more.
Okay... read this twice, but very tired... if I'm rambling or don't make sense, please let me know......