By lumpy do you mean lots of small lumps or larger almost fatty deposits? There are a few things which might help. As
@himtoo posted above, making sure the injection sites are spread out is really useful. I find giving my larger doses (for me this is long acting) into my legs and bum is easier as there is more tissue for it to disperse in so it doesn't create lumps under the skin. Another thing to make sure your mum is doing is changing her injection needles every time she injects. She might already be doing this but if not, a good habit to get into. Finally, have you mentioned this to your diabetes team? They may look at changing the needle lengths, but if she's already on 4mm not sure if there is anything to change to. Hope this helps.