Well, all I can tell you is what you already know, that the twitches/tremors and other symptoms she is already exhibiting are the start of a severe calcium deficiency, and possibly other nutritional deficiencies, and the start of MBD. I don't know if the Reptisun 5.0 you already have is a long 18" tube or a compact/coil bulb, but if it's a compact/coil bulb then she's gotten absolutely no
UVB light ever. That's what's happening to her. Would I wait another second to go buy a proper, long UVB tube for her? No, I wouldn't, because at her age this is going to end up resulting in multiple problems for her, one of which could be permanent physical disabilities, like paralysis in her back legs, keeping her front legs folded underneath her, lumps in her spine, etc.
At 4 months old she is supposed to be doing most of her growing and developing right now, and she's not able to do any right now. She is not absorbing any nutrition or supplements you're giving her at all without any
UVB light. So she could end up very stunted in growth, in addition to the MBD and physical disabilities.
I have a year and 4 month old male rescue dragon that I got at 7 months old from his first owner, and he had no
UVB light at all until I got him. He's very small, only 15" long and will be for the rest of his life. His front arms are both always folded underneath him, he has a lump in his upper spine, his back legs are weak and he cannot poop unless he's in warm water. He has trouble walking on flat ground without falling. I have been able to reverse some things, like his appetite is great now, and he no longer flips onto his back while just walking on flat ground, and he no longer has tremors, but that's about all I could help with, and I got him under a very strong, 22" Reptisun 10.0 T5 High-Output UVB tube in a 24" T5-rated fixture for it (both of which you can buy on Amazon.com right now for around $50 shipped by the way, much cheaper than in a store, you'll pay more than that in any pet shop for the much weaker and shorter 18" Reptisun 10.0 T8 UVB tube and a fixture for it, so whatever you do I recommend that you order it online, Amazon.com seems to be the cheapest place for them that I've found). He's been under the 10.0 T5HO UVB tube for 14 hours a day since the day I got him, he's been in the correct temp ranges, fed high-calcium BSFL as his live staple insect every day, dusted in calcium powder and multivitamin powder, and though he's much better than he was, he's still forever stunted and disabled. I can't fix that, and it's frustrating. But it just goes to show how much damage a lack of proper and strong
UVB light for at least 14 hours every day can and will do to a desert reptile, especially a Beardie.
I understand your financial issue, I can only give you the facts and what I would do, which is someone or multiple people wouldn't be getting Christmas presents so that I had $50 to order the proper T5 UVB tube and fixture for it on Amazon.com today. That's just me, and not meant to be pressure onto you or be judgmental or critical of you, just what I would do, only because I live with what a lack of proper
UVB light does to these guys every day.